Add function for retrieving column names from table filters.
Function for parsing column names from a patsy formula.
Add toolz to travis.yml and setup.py.
Add "not" to reserved words in columns_in_filters.
Add .columns_used methods to regression model classes.
Add .columns_used methods to LCM classes

Since LCMs deal with three tables (choosers, alts, and those merged)
there are actually four total methods:
- .choosers_columns_used
- .alts_columns_used
- .interaction_columns_used
- .columns_used (union of all the above)
Merge pull request #49 from synthicity/model-columns

Get columns used by a regression and LCM models