Implemented drop_foreign_key.

This is my shot at `drop_foreign_key`, something I have missed in sequel for ages. In fact, I have been using monkey patch which added the same functionality for several years, but it was mysql specific. The addition of `foreign_key_list` was exactly what was needed so I could streamline the patch so [other people who want it](https://groups.google.com/forum/?fromgroups=#!topic/sequel-talk/SlKQQCbnKfQ) might benefit from this as well.