[WIP] Rackspace Auto Scale: groups

[bison]

Adds a package for Rackspace Auto Scale groups.

[coveralls]

Coverage increased (+0.03%) to 80.177% when pulling 86ba0bd on bison:autoscale-groups into d62a69f on rackspace:master.

[bison]

Getting started on the final bit of adding Auto Scale support, but a few things are a little awkward already.

First, when you GET an individual group, the response consists of a UUID, configuration object, state object, launch configuration object, and a list of scaling policies. When listing groups, the groups only contain the UUID and the state object, so the others will be empty or nil. That seems kind of confusing, but I'm not sure what can be done about it.

Second, you don't update a group resource directly, you update its configuration at its /config URL, which you can also GET separately from the group. So, does it make more sense to have GetConfig and UpdateConfig (with an ConfigUpdateOpts type) methods in this package, or just have groups.Update apply to /config?