Python3 PR, updated

I took PR #4 and updated it:

- Properly converting str/bytes.
- Updated tox and Travis to build for py26, py27, py33 and pypy. Removed py32 (it doesn't support u"" prefix for strings).
- Absolute and relative imports working as intended.
- Implement missing methods for MutableMapping (__delitem__, __iter__, __len__).
- Using feature detection instead of version checking between py2 and py3.