Revert mocha back to 1.18

Newer versions cause karma test runner to freeze.
Support custom User-based models

Support application using a custom User-based model for authentication,
e.g. `Customer` based on `User`.
Fix ngdoc comment for related method

Before this change, the generated ngdoc included method name
in `methodOf` directive, e.g. `@methodOf User.accessTokens.create`.

This commit changes the ngdoc to correct `@methodOf User.accessTokens`.
Merge pull request #58 from strongloop/feature/support-custom-model-class

Support custom model class