Read groups and groups/id endpoints

Hey folks,

I'll keep adding PR's that y'all can review, add to and/or tear apart them seeing @paulgibbs mentioned on Twitter that you're both flat out which I can relate to seeing I'm like that 99% of the time :smile: 

This PR should add reading endpoints for /groups
![groups](https://cloud.githubusercontent.com/assets/1377956/4679584/37c0d192-5603-11e4-98c0-c637f8dd42f0.png)

And reading endpoints for /group/id
![group](https://cloud.githubusercontent.com/assets/1377956/4679591/4f147326-5603-11e4-87cb-745d676c2048.png)