IN operator bug fix

Fix to the in operator not allowing more than two entries

New features

• Add support for count distinct
• Add support for coalesce
• Add support for multi table joins

Add Support for joins with overlapping columns

This release adds support for joining on tables that have some/all of the same column names

ENH: Double quote support

• Adds support for spaces in column names using double quotes

Ibis update and minor bug fixes

• Now uses ibis 1.4
• Added Python3.8 to CI
• Checking for empty values is now supported

Fix setup.py readme type

Merge pull request #45 from zbrookle/fix_pypi_readme

Fix pypi readme

Hopefully last documentation release

More documentation fixes on PyPi

Minor documentation update

Updates to PyPi documentation

Major bug fix

Fixes bug in certain backends where columns couldn't be used in where clause unless also selected.

For example,

SELECT A,B FROM demo_df WHERE B > 4 LIMIT 1

would throw an error with the pandas backend.

New features and bug fixes

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