Rename dc:format property to dcformat (fixes #11)

The `dc:format` property definition used the name `:format`, which conflicts with the [Kernel #format method](http://www.ruby-doc.org/core-2.1.5/Kernel.html#method-i-format). This changes the property definition to use `:dcformat`.
Do we know what, at this level, would fail?  Possible to write a test that fails for `format`, which could be run against all properties?