TST Import sqlalchemy on Travis.

DOC add docstrings to read sql

ENH read_sql connects via Connection, Engine, file path, or :memory: string

CLN Separate legacy code into new file, and fallback so that all old tests pass.
TST to use sqlachemy syntax in tests

CLN sql into classes, legacy passes

FIX few engine vs con calls

CLN pep8 cleanup
add postgres support for pandas.io.sql.get_schema