AllVerbsMixin: call a method for all HTTP verbs

This refers to issue #97.

The dispatch method will extract the attribute specified by the class attribute "all_handler", the default value of which is "all". The dispatch method then calls this method, without checking what the HTTP verb is. If "all_handler" is not specified (technically if it evaluates to False) then ImproperlyConfigured exception is raised.

Tests are in tests/test_other_mixins.py::TestAllVerbsMixin.

I was able to run the tests using py2.7-d1.4, py2.7-d1.5, and py2.7-d1.6. I don't have the other two interpreters installed on my system. I will check them soon.