- Query.distinct() now accepts column expressions

- Query.distinct() now accepts column expressions
  as *args, interpreted by the Postgresql dialect
  as DISTINCT ON (<expr>). [ticket:1069]
- select.distinct() now accepts column expressions
  as *args, interpreted by the Postgresql dialect
  as DISTINCT ON (<expr>).  Note this was already
  available via passing a list to the `distinct`
  keyword argument to select(). [ticket:1069]
- select.prefix_with() accepts multiple expressions
  (i.e. *expr), 'prefix' keyword argument to select()
  accepts a list or tuple.
- Passing a string to the `distinct` keyword argument
  of `select()` for the purpose of emitting special
  MySQL keywords (DISTINCTROW etc.) is deprecated -
  use `prefix_with()` for this.
- put kw arguments to select() in order
- restore docs for _SelectBase, renamed from _SelectBaseMixin