on_model_change and on_model_delete hooks

Fixes for boolean fields

Example here: https://gist.github.com/3714266

fix bug in form.py wen checking callable_default

    raise TypeError("Boolean value of this clause is not defined")
TypeError: Boolean value of this clause is not defined

see
http://www.sqlalchemy.org/trac/wiki/06Migration#AnImportantExpressionLanguageGotcha

Update README.rst

Fixed #53. It is now possible to specify field conversion classes for sqlalchemy and peewee backends.