Honor the :include option when running with :drb

In the existing code, the drb test runner did not honor the :include option, so this commit adds it in.

In deference to 1f76e, which removed the automatic inclusion of test folders from the drb runner because it was breaking spork-minitest (Issue #72), the commit maintains the existing behavior of the drb runner and does *not* include :test_folders by default.  They can still be expressly included by putting them in :include.