add `Database.change`
Database.change and Database.select_db
add group by
add test for models.groupby
add groupby to models; add its tests
changes for 0.3.6
add having
tests for having; changes for v0.3.6
add distinct keyword
docs for `having` and `groupby`
tests for distinct; models.having
changes for v0.3.6
doc for distinct
ready for 0.3.6
Merge branch 'dev'

* dev:
  ready for 0.3.6
  doc for distinct
  changes for v0.3.6
  tests for distinct; models.having
  docs for `having` and `groupby`
  add distinct keyword
  tests for having; changes for v0.3.6
  add having
  changes for 0.3.6
  add groupby to models; add its tests
  add test for models.groupby
  add group by
  Database.change and Database.select_db
  add `Database.change`