BUG/TST: Allow generators in DataFrame.from_records

With 0.12 and development version it's not possible fo feed a generator to Dataframe.from_records if nrows = None, it leds to a TypeError exception see: http://stackoverflow.com/questions/18915941/create-a-pandas-dataframe-from-generator

It's no clear whats nrows do, it's not documented.

I've create a PR with a simple fix without touch nrows, and two tests for generator data input.
