Improvements, bug fixes and tests in MySQL port; 

Test psql and mysql separately, depending on platform support. All the logic is in `test.sh`, and subtests are in `test/`.

Added a `ndbloader` for mysql cluster data loading. The functionality is in DataLoader, but it is not yet used in DeepDive pipeline.
 
Updated all unit tests and integration tests to fully support MySQL.

Added a new integration test: spouse example, only for MySQL.

It was originally commented out, introduced in cc18ddc2d0421fce5c1152f6762a8fec9c0782ca on March. I just cleaned this up.