Update jinja2_hamlpy.py

- Modifies style to match PEP8
- Uses `not name` instead of `name is None` ('' should be an invalid name as well, right?)
- Refactors available HAML file extensions to a class-level variable.
Merge pull request #1 from jgeewax/patch-1

Modifies style to match PEP8
Uses not name instead of name is None ('' should be an invalid name as well, right?)
Refactors available HAML file extensions to a class-level variable.