API: update SQL functional api (GH6300)
First draft for the functional api part of #6300.

OK, I think this is fine now. I only changed the toplevel api (so `read_sql` -> `read_sql_query`, `read_table` -> `read_sql_table` and a new `read_sql` as wrapper around both), and not yet the methods of the PandasSQLAlchemy objects (as we did not yet discuss that).

Somebody care to do a quick review? @hayd @jreback @danielballan @mangecoeur
 Other than those minors docstring suggestions, looks great to me.
 Looks good (sorry, thought I commented earlier!)
 Thanks for the comments! Adapted the summary lines of the docstrings a bit. 
Merging now.
