Update SublimeFiles plugin to work with ST2 and ST3.

Hi,

Apparently I've come up with similar plugin to yours. In order to avoid duplicating plugins, here's my pull request to make yours work with both ST2 and ST3.

Changes:
- Updated documentation + added little gif showing plugin in action
- A couple of small fixes 
- Code fixes + adjustments to make it more PEP8 compliant 

More: https://github.com/hmml/SublimeFiles/tree/py3-totalfix