Delegate wrapping functions for prepending and appending to form fields to the frameworks

Per comment in #169, there's a lot of implicit Twitter Bootstrap 2 structure and markup in the wrapping code. This pull request deals with that by pushing that functionality down to the frameworks.

All existing tests pass (one small tweak needed for a removal of a spurious space when appending), though the tests are a little light in this area. If you like this pull request, I'll add some tests that address this specific functionality (which has already been tested by use with each of the frameworks).

This pull request doesn't support Bootstrap 3. I have that too, but need to merge it with code changes applied yesterday before sending as a pull request.