Add the add_property method

Adds support for a public add_property method which exposes adding
a property to a Format object dynamically while still doing the
right thing with to_hash and to_json.

Took forever and a day for us to figure out how to properly get this 
under test coverage. @jlsuttles it feels funny to put this test for 
something in the Format class into the spec for a Collection but
still seemed like the best spot based on the other specs we found.
We did try creating a format_spec but ran into all kinds of issues so
left these tests where they were. If you have any other input let us
know. We also commented out the pending tests at the bottom of 
this file for now since they were failing anyways.