TEST: add basic postgresql tests

TEST: add postgresql to travis
TEST io.sql: sqlite tests to seperate class

One base class for tests with sqlalchemy backend. So test classes
for mysql and postgresql don't have to overwrite tests that are
different for sqlite.
Merge pull request #6301 from jreback/mi_indexing

ENH: per axis and per level indexing (orig GH6134)
Merge pull request #6316 from jorisvandenbossche/sql-postgresql-tests

TEST: add basic postgresql tests
BUG: Bug in setting complex dtypes via boolean indexing (GH6345)