Fixes #221.

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

- Make a test in Py3k actually use StringMixIn instead of str.
- Minor cosmetic fix.

Remove some extraneous whitespace in string_mixin.py.

- Skip CTokenizer tests if CTokenizer is not available.
- TestStringMixin: Don't make assumptions about default encoding.
- Add urllib stuff to mwparserfromhell.compat.
- Fix compat.py's line endings.
- gen.next() -> next(gen)
- assert*Equals() -> assert*Equal()

Still need to do some py3k-only ones.

Also reorder imports in nodes.__init__ to fix a minor bug.

(Don't mind me, just developing software on Amtrak on the way to
Wikimania).