Python 3 Support

This pull request adds python 3 support into wagtail.

The following changes have been made:
- Removed all unicode literals 
- Fixed relative imports
- Used six for string type checking and importing StringIO/urllib
- Converted all __unicode__ methods to __str__ (and used djangos python 2 compatibility decorator)
- Changed syntax of print statements
- Added metaclass to Page object in a python 2/3 compatible way (using six)

There are currently a couple of issues that need to be addressed before merging this PR:
 - There are no pypi releases of django-modelcluster, django-libsass and django-compressor with Python 3 support (but these all support it in their github repos)
 - Needs more testing
 - Needs tox setup (for travis)

To test, please use my python3 branch at https://github.com/kaedroho/wagtaildemo/tree/python3.