duplicate pandu-rao's doc fixes in the readme
Merge remote-tracking branch 'upstream/master'
Added guards to handle None value in Date and DateTime column to_python conversion
Added guard to handle None value in Date column to_database conversion
Add testcases for guards for None in date, datetime conversion
Fixed bug in test case and expanded test case to include values_list
Merge branch 'master' of github.com:pandu-rao/cqlengine into pull-112