pytest is the preferred way to write and run unit tests these days and
it has a cleaner interface - so lets switch to it. The tokenizer tests
especially are much easier to read/understand.

This was mostly done with find/replace regexes and then cleaned up
manually.

Fixes #221.

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.

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.

* 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.