This release implements extensive testing for postgres and sqlite. It also sets up (but skips) pandas unit tests.
It follows from this PR: https://github.com/machow/siuba/pull/36
SQL Improvements:
* fix distinct when following an arrange (65)
* move collect and show_query into dply.verbs (23)
* mutate now doesn't put redefined columns at end of table (42)
* full join gets all values in joining columns, even if unique to only 1 table (57)
* complete implementation of joins, including anti_ and semi_join (58, 55, 53)
* allow mutate to accept only scalar values, e.g. mutate(a = 1). (39)
* handle arbitrary siu expressions inside arrange (30)
* track arrange expressions, in order to allow cumulative functions (30)
* clearer messaging that summarize can't refer to just defined column (49)
* summarize, and mutate better identify when referencing a variable requires a CTE (46, 45)
* allow keyword args to group_by (52)