Python 3 fixes

Relates to issue #1 (Python 3 support)

Made setup.py work with Python 3 (convert binary to text).

In Python 3 the SCSS source code must be decoded from binary to text before processing. The latest compressor passes a charset argument to our filter, which will be used. If not passed, it will default to utf-8.

Note that Python 3 support currently depends on the latest (master) compressor and a pyScss with this fix incorporated (pull request submitted): https://github.com/wojas/pyScss/commit/e4cf419509f3c568c108c3196e2fda50bcc37536

NOTE: Tests related to the sprites fail for me, both on python 2.7 and 3.4. The hash is different from what's expected.