Compatibilty (windows, python versions)

Hi again,

Having started with a compat_win branch for windows compatibility (especially about `os.sep`), I've extended its scope and added a tox file for automated testing across environments (feel free to drop the tox file if you don't want it, I simply find it handy to generate virtual environments).

Btw, half of the commits here are already included in the import_sprite branch (PR #284).

Main comments:
- ensured consistency in hashes between windows and unix when they are generated from (relative) paths
- ensured that every `unix` input file path was translated into a `windows` one, and that every URL was generated with forward slashes only
- replaced `codecs` by `io` to import `open` on python 2 (`codecs` causes issues with linebreaks)
- added requirement for `enum34` package when using python < 3.4
- directly executes `scss_meta.py` in setup.py, so there is no need to install six locally as `scss/__init__.py` is not imported anymore
- py26 compatibility: there was a combined `with` statement in `scss/functions/fonts.py`, which I replaced by 2 nested `with` statements. It may also be considered to drop support for py26

Observations:
- On py32 and windows, 2 tests fail, as the returned mimetype for png is `image/x-png` instead of `image/png`. Didn't dig into that, can live with it