BUG: single group series should preserve group name
Extract currently doesn't preserve group names when a series is returned. This
PR fixes that.

@jreback good 2 go after a doc blurb?
 sure
 what's the priority of travis jobs it seems like i have to wait for a long time sometimes and then other times i don't wait that much. really frustrating 
 this passed a while ago and it seems like it'll never run on master

https://travis-ci.org/cpcloud/pandas/builds/26600221

merging
