Commonmark

- The CommonMark spec has been updated to 0.25.
- The HtmlRenderer has been refactored based on upstream changes in commonmark.js.
- Python 2 fixes from eric-wieser
- Contributions from funkybob and hyperknot

- commonmark.py now supports Python 2.6.
- The CommonMark spec has been updated to 0.24.

- Fixed a UnicodeEncodeError when parsing unicode entities on
Python 2. As a result, commonmark.py now relies on the "future"
module in Python 2, as documented in setup.py. This can be found on
pypi: https://pypi.python.org/pypi/future

- Fixed an IndexError exception that occurred when input string
was empty.

- commonmark.py now complies to the 0.23 CommonMark spec
http://spec.commonmark.org/0.23/
- The ExtensionBlock has been removed in this release, since
the parser has been rewritten.
- Added a compatibility fix for Python 2.6, but this version
of Python still isn't really supported.
- `HTMLRenderer` has been renamed to `HtmlRenderer`.
- `DocParser` has been renamed to `Parser`.

- Random bug fixes
- Internal code structure changes
- Compatibility fixes for Python 3