GBQ: Updated Documentation, and added method to generic.py
Added some documentation for GBQ module

i led u astray

move to_gbq to core/frame.py (from core/generic.py)
as you only allow data frames (no series or panels) 
 @jacobschaer 

actually....instead of moving to `core/frame.py`, why don't you just check that the `obj.ndim==2` and otherwise `raise NotImlementedError`, that way you can easily support a series at some point in the future if you want, and you get a nice error message (and not `AttributeError`)
 Not sure if that's the best choice. If we did choose to support it later,
introspection on the object in this version would suggest that this exists
and is supported.
 @jacobschaer reverting back......so move `to_gbq` from `core/generic.py` to `core/frame.py`...makes the most sense (we have this with `to_stata/to_csv/to_sql` as they don't make sense generally for non-frames).
 @jacobschaer can you fixup?
 @jreback - Sorry about the delay. Docs should be fixed up, and the `to_gbq` method is in `core/frame.py`.
 just a small change; pls squash and i'll merge this
 @jreback - Fixed and squashed.

@jorisvandenbossche - Not entirely sure what needed to be done, but I think I did what you asked.
 @jacobschaer Indeed, you did what I meant!
