Start of Praxis::Collection and Praxis::MediaTypeCollection

Signed-off-by: Dane Jensen <dane@rightscale.com>
Skip MediaTypeCollections in doc browser index.

Signed-off-by: Dane Jensen <dane@rightscale.com>
Tweak Praxis::Collection to be pure sugar, and not a subclass of Attributor::Collection.

Signed-off-by: Dane Jensen <dane@rightscale.com>
Flesh out MediaTypeCollection specs a bit more with validation specs.

Signed-off-by: Dane Jensen <dane@rightscale.com>