Improved documentation strings.

Refamiliarizing myself with the code.  Added some todos.
Remove offset argument from glmnet.__init__
Add _check_if_fit and _check_if_unfit.

These two methods are added to GlmNet.  Checks for fittedness and
unfittedness added in subclasses where appropriate.
Fix max_lambda calcualtion in ElasticNet when sample
weights are used.
Add tests dir and initial tests for ElasticNet.