Added version indicator to admin dashboard

This pull request does the following things:
- Adds a PEP396 module version number to wagtailcore (http://legacy.python.org/dev/peps/pep-0396/)
- Uses this version number from docs and setup.py (now only one place to bump the version)
- Prints this version out onto the dashboard

TODO:
 - [x] @davecranwell  Make the version indicator look nice