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.
Improved validation for box constraints.
Add tests for data validators.
Improve validate_weights.

Sample weights must be non-negative.
Improve validate_excl_preds:

This one was a little broken.  Add tests for valid shape, and valid
entries.
Improve validate_box_constraints.

Add check for validity of intervals.
Add examples of correct use in validate method tests.
Change coefficients to _coefficients.
Add col_names attribute to ElasticNet.
Add get_coefficient methods to ElasticNet.
Add describe method to ElasticNet.
Fix small bug in last commit.
Clean up LogisticNet a bit.

  Remove dependence on sklearn.standardize.
  NotImplementedErrors.
  Better handling of y in .fit.
Remove dependence on sklearn.standardize in enet.
First tests on LogisticNet.
Add test of ridge regression case.