Integrate with Mull for mutation testing

[isaacbrodsky]

Adds support for running the tests under Mull, a mutation testing framework. Mutation testing helps evaluate the quality of the test suite by seeing how well it catches randomly inserted errors in the program under test.

• This is (in another form, for branches specifically) is part of the SQLite test methodology
• I had to modify the tests that accept data files to optionally read from the file themselves, otherwise the tests are unmodified.

A full run can take quite some time so it probably does not make sense to run this on every PR.

[coveralls]

coverage: 99.662%. remained the same — isaacbrodsky:mull-integration into uber:master