New --repair option for configurable test runner

The option can take several values:
 - "show" tells the tests to display the changes that are to apply to the
    database
 - "1", "yes", or "do" tels the tests to make the changes themselves
Tests must implement the Repair interface to understand --repair
Merge pull request #16 from muffato/add_repair_option

New --repair option for configurable test runner