Fix send_message to support passing generic data.

If you pass a string, old behaviour is preserved, otherwise, if you pass
a dict, it's passed straight as the data.
Support sending bulk messages to a queryset.

Based on code from:
https://github.com/jleclanche/django-push-notifications
Merge pull request #15 from tasn/develop

Add support for needed real life features
Move DeviceQuerySet to the model manager
Move code to the common function
Refactoring gcm api
Fix typo in help text
Handle errors in GCM responses.

Added a "mark_inactive()" function to the Device class that is
override-able and is called now instead of is_active=False.
mark_inactive() is now called if the appropriate errors are returned.

This is according to:
http://developer.android.com/google/gcm/http.html
Merge pull request #17 from tasn/error_handling

Handle errors in GCM responses
Simplify the code
Add tests
Call post send for each chunk
Don't check explicit device ids
Add coverage
Add management command test
Add tests
Add missing license
Add tests for the admin view
Add coveralls badge
Bump version