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 want to change this in tho context?  My assumption had been that we would keep the property names aligned with the JSON-LD expressions, but this feels a bit like the tail wagging the dog.

Thoughts?