[mllib] [python] Add LassoModel to __all__ in regression.py

Add LassoModel to __all__ in regression.py

LassoModel does not show up in Python docs

This should be merged into branch-1.3 and master.

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #4970 from jkbradley/SPARK-6253 and squashes the following commits:

c2cb533 [Joseph K. Bradley] Add LassoModel to __all__ in regression.py

python/pyspark/mllib/regression.py

@@ -21,8 +21,10 @@ from numpy import array
 from pyspark.mllib.common import callMLlibFunc, inherit_doc
 from pyspark.mllib.linalg import SparseVector, _convert_to_vector
 
-__all__ = ['LabeledPoint', 'LinearModel', 'LinearRegressionModel', 'RidgeRegressionModel',
-           'LinearRegressionWithSGD', 'LassoWithSGD', 'RidgeRegressionWithSGD']
+__all__ = ['LabeledPoint', 'LinearModel',
+           'LinearRegressionModel', 'LinearRegressionWithSGD',
+           'RidgeRegressionModel', 'RidgeRegressionWithSGD',
+           'LassoModel', 'LassoWithSGD']
 
 
 class LabeledPoint(object):