REFACTOR allow to use postgres

Add postgres support for tests.

Later, all major databases will be supported through travis, but let
just add environement switches for issue at hand with postgres.

Needed for #15
Merge pull request #17 from oelmekki/allow_postgres_usage

REFACTOR allow to use postgres
Revert "Merge pull request #17 from oelmekki/allow_postgres_usage"

This reverts commit 5acb07019aa04d0f88e0e1c0d7f55681d13414a0, reversing
changes made to af8dbca25771aab6f062b1f1f3fd144364323573.

This is to undo temporary changes to allow running tests on
multiple databases, which will be covered by the configs
for TravisCI in forthcoming commits.
Configure the library for running on TravisCI

Removed dummy apps (we load the schema and models
necessary for testing). Added .travisci.yml with
a variety of environments to test. Added gemfiles
for rails 3, 4, and edge.