Google Quote shouldn't return a DataFrame and should be documented

[femtotrader]

Hello,

In [3]: pdr.get_quote_google(['MSFT', 'F'])
Out[3]:
          0
MSFT  51.30
F     12.47

Google Quote shouldn't return a DataFrame but either a Series or a dict

Moreover some doc is necessary.

Pinging @gliptak

Kind regards

[femtotrader]

GoogleQuotesReader should also be available through DataReader function

maybe we should use google-quote as data_source

[gliptak]

@femtotrader Added docs with #188

[gliptak]

Implemented.

[femtotrader]

Thanks @gliptak for this also.
I'm closing this issue as your PR is merged.