Replaced default Django's User model with custom one.
Added new "get_model" util to "core" app.
Fixed signals and tests handling of custom user class.
Updated required Django's version to 1.6.0.
Use of Django 1.6.0's "SuccessMessageMixin".
Fixed notification follow and unfollow views (also improved signals and docs).
Added some tests for SubscriptionForm.
Plugget sources are now registered by title instead of func UID.
Whitespace are now stripped out on title validation.
The value of Plugget "source" field is now handled in two steps:

1) Firstly try to use it as the UID of a registered source.
2) Then (if no source is found)  try to use it as a function path.