Add custom mapping to the repositories index

For a more seamless integration into Rails I have added a custom mapping
using the type 'repository' and adding the repository type itself with a
subtype property.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Declare _type attribute explicitly

I have changed the type declaration by using _type now. This way I can
also use the type attribute.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>