Fixes #435.

All file-based tests are now defined as unittest test cases via a
metaclass which walks a directory and builds a unittest for each pair
of test files.

To run the tests just run `python -m unittest discover tests`. Or use
tox as the tox config has been updated to run the new tests and all nose
specific code has been removed.

The test generator tools have been removed as well. If any changes or
additions need to be made to tests, they should be implemented using
the new framework rather than with the file-based tests. Eventually,
only the PHP and pl tests should remain as file-based tests.

As a part of the Markdown lib, test tools can be used by third party
extensions. Also keeps test dir clean as it only contains actual tests.
More work in this vein to come as the need for Nose is removed.

Tests are defined as Unittests rather than in text files  allowing
features to be more easily broken into units and run individually.
Based completely on standard lib unittest with no external dependencies.
Use `python -m unittest tests.test_syntax` to run.

Pulled some tests from https://github.com/karlcow/markdown-testsuite.
Many more test units to pull from that source. As we encounter the need
to edit an existing textfile-based test, or add a new test, a new test
should be created with this framework and the old test should be
deleted. Also need to delete existing testfile-based tests which are
covered in the new tests included here.