added types and groups endpoint

[argvader]

Thanks for doing this work! We are using this code as a base for interacting with marathon and need the groups endpoint and additional types added.

[argvader]

Many of the types are also included in the PR by antonlindstrom

[jbdalido]

Hi, many thanks for working on it, i'm catching up, i'm a bit late (a lot). I'm merging everything with caution right now, and i'll run full tests tomorrow, and then push everything.

I'm also starting to write tests to have a more stable release. All pr are welcomes if you want to help :)

Thanks again.