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.

Do we need this file? (2)