Support pandas.Series in the Dataset constructor

[shoyer]

Follow up on #677

cc @MaximilianR

[max-sixty]

I think this is why it doesn't currently work: http://stackoverflow.com/questions/35442924/how-does-pythons-dict-constructor-handle-mappings

Tangentially related to: pandas-dev/pandas#12056 - i.e. from the pandas side, should Series be Mappings?

We could add a separate path for Series in the dataset constructor, or even monkey patch ChainMap. Neither seems that elegant

[shoyer]

I would be perfectly happy with adding something like this to the Dataset constructor:

if isinstance(data_vars, pd.Series):
    data_vars = dict(data_vars)