TEST: add basic postgresql tests

@mangecoeur I just copied and adapted the mysql tests and provided some `SQL_STRINGS` that follow the postgresql dialect. Is this what you had in mind how other sql dialects would be added to the tests?  

@jreback I now added it to requirements-2.7.txt. Should I add it to all test environments? Or if only one, is this the correct one?

@mangecoeur Is it possible that there is a `pymysql` missing in the requirement files to run the MySQL tests (they are all skipped on travis)?

At the moment, there are two tests failing with postgresql (see https://travis-ci.org/jorisvandenbossche/pandas/jobs/18576746).