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.

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

- 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()

Add from __future__ import unicode_literals to a few files.