Add a fallback from 'test:prepare'

[MixmasterFresh]

These changes fix an issue with rails-perftest as mentioned in issue #24. It checks to see if there is a rake task for 'test:prepare', and if there is it will continue as it would normally. Otherwise it falls back to the 'db:test:prepare' rake task. This passes all of the tests (which need to be improved) and I tested it on a few rails apps, and it worked just fine. Resolves #24.

[senny]

@TheAustinSeven can you squash your commits?

[MixmasterFresh]

The commits have been squashed. Good catch.

[MixmasterFresh]

This should be passing, the error seems to not be with the code itself but rather with running bundle install. If it were rerun, it would likely pass.