ENH #4163 Use SQLAlchemy for DB abstraction

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

WIP: cleaup of sql io module - imported correct SQLALCHEMY type, delete redundant PandasSQLWithCon

TODO: renamed _engine_read_table, need to think of a better name.
TODO: clean up get_conneciton function

ENH: cleanup of SQL io

TODO: check that legacy mode works
TODO: run tests

correctly enabled coerce_float option

Cleanup and bug-fixing mainly on legacy mode sql.
IMPORTANT - changed legacy to require connection rather than cursor. This is still not yet finalized.
TODO: tests and doc

Added Test coverage for basic functionality using in-memory SQLite database

Simplified API by automatically distinguishing between engine and connection. Added warnings