Added staticfiles panel class.
Actual show the number of used static files in panel title.
Another pass over the staticfiles panel.

- adds some docstrings
- adds some basic tests
- adds the staticfiles panel to the default list of panels
- fixed a pluralization bug in the template
- refactored some things into own methods for easier testing
Use django-discover-runner on older Djangos.
Added the staticfiles panel to the docs.
Added django-discover-runner to travis config, too.
Made a test less Django version dependent.
More test relaxing.
Use collections.OrderedDict if available.
Some imports.
Use enable_instrumentation/disable_instrumentation instead of module level monkey patching.
Storing the original on module level.
Merge pull request #494 from django-debug-toolbar/staticfiles

Added staticfiles panel class.