Add some table tests to increase coverage. Also reorder some tests.

For wiki syntax tables, add `wiki_style_separator` as an attribute
for the Tag node. Also reorder `closing_wiki_markup` property and tests
to match its place in the constructor.

Table tags no longer self-closing. Rows and cells now contain their
contents. Also refactored out an `emit_table_tag` method.
Note: this will require changes to the Tag node and possibly the builder,
those changes will be in the next commit.

Remove style test to properly implement implicit style closes later.

Fix problem in which invalid table attributes were being parsed
incorrectly. Added tests.

Fix problem in which fake table closes were causing a problem inside
cells. Changed inline table handling to fix this.

Padding now included on all wiki table cells. With wiki table cells
that include attributes, `wiki_markup` is also included (unchanged).

Removed the `StopIteration()` exception for handling table style
and instead call `_handle_table_cell_end()` with a new parameter.
Also added some random tests for table openings.

Changed row recursion handling to make sure the tag is emitted even
when hitting recursion limits. Need to test table recursion to make
sure that works. Also fixed a bug in which tables were eating the
trailing token. Added several tests for rows and trailing tokens with
tables.

Tables and rows use newlines as padding, partly because these characters
are pretty important to the integrity of the table. They might need
to be in the preceding whitespace of inner tags instead as padding after,
not sure.

Added support for allowing different wiki syntax for replacing the opening
and closing tags. Added for table support.

Started styling attributes for table row and table start. Still not entirely
sure about this, definitely need to make changes regarding padding.

Support for header cells was mostly in already, just needed minor
changes. Added two tests as well.

Started support for parsing table style attributes. I suspect some
of this is incorrect, need to add more tests to see.

Added another stack layer for tokenizing table cells because of
styling/correctness of implementation. Added many tests cases.

Started parsing table support and added the start of table support.
This is a big commit (ugh) and it should probably be split up into
multiple smaller ones if possible, but that seems unworkable as of
right now because of all the dependencies. Also breaks tests of
CTokenizer (double ugh) because I haven't started table support there.

May want to pick line by line on this commit later but I need to save
my work for now.

* Parameters with non-integer keys can no longer be created with
  showkey=False, nor have the value of this attribute be set to False
  later.
* Calling Template.remove() with a Parameter object that is not part of
  the template now raises ValueError instead of doing nothing.
* Added tests for HTMLEntity._unichr() being called with out-of-range
  codepoints.
* Added tests for Tag.__children__() and Tag.__showtree__() involving
  attributes that have no values.

* Removed unittest2 dependency on Python >2.6.
* Moved discover_tests.py into tests/.
* tokenizer.c: Fixed errors noted by -Wshorten-64-to-32.

Requested by Σ.

Reported by Σ.

**Breaking change**; closes #51.

Bump version to 0.4.dev.