Fixes bug in certain backends where columns couldn't be used in where clause unless also selected.
For example,
SQL SELECT A,B FROM demo_df WHERE B > 4 LIMIT 1
would throw an error with the pandas backend.
0.2.0
Changes:
* Data classes are now used internally so Python 3.6 support is dropped * Window functions are now supported (see readme for syntax) * Issue with joining based on multiple columns is fixed