Use codecs package and context manager for reading

open(..., encoding=...) is only available with py3k, but the codecs
package is around forever. This way we always end up with a unicode
object holding the contents of either README.rst or LICENSE. Use a
context manager (with keyword) for extra clean file descriptor closing.
Fix setup.py PEP-8 issues (and catch with tox)
Support semicolons in mixin arguments

Check https://github.com/twbs/bootstrap/commit/c017e19ed236