Add ApiToken model for persisted token-based API access

This is currently limited to user associated API tokens since Serrano only
supports authenticated users and clients that support cookie-based sessions
throughout the exposed resources.
Add support for extracting the token from request headers

Clients can now set the `Api-Token` header instead of supplying the
token as a GET or POST parameter.
Remove non-applicable third clause from LICENSE
Fix import of Log model in test cases

The import of the Log model was changed in cbmi/avocado@78cfec59 to not be
imported to avocado.models. The full path to avocado.events.models is now
being used.
Add migration for ApiToken model
Only use ApiToken model if serrano is installed

This adds a condition in the authentication backend to only use the
ApiToken model if Serrano is installed in INSTALLED_APPS. This is to
maintain backwards compatibility since this was not previously a
requirement.
Ignore auto-generated migration files from flake8
Merge pull request #122 from cbmi/issue-109

Support for user-persisted API tokens