Bring ``catsup clean`` command back.

Deleted by mistake.
Improved static file support

+ Add ``config.config.static_source``
+ Add ``config.config.static_output``
Fix a bug on ``catsup.reader.markdown``
Don’t copy folder when the source folder is not exist.
Prepare for rewriting tests.

Remove all the test.
Make ``catsup server`` work properly.

Yes, I fixed a bug..
Add txt format support(Untested)
Add tests for ``get_reader`` func
Escape txt post’s content
Only give posts archive and tags.
Forgot to import Post in ``catsup.reader.txt``
Move ``catsup.generator.models`` and ``catsup.generator.utils`` to top level
Support Non-meta post
Update change log
Add tests for catsup.parser
Add more tests
Support customize permalink for post
Add more tests
Add support for HTML format post.
Submit coverage reports to coveralls.