Add `feature_indices_` attribute to mapper

[dukebody]

The attribute indicatesthe mapping between input and ouptut variables (see added docs). I'd like someone to give +1 before merging. :) @asford ?

Differences with implementation in https://github.com/paulgb/sklearn-pandas/pull/54:

• Instead of storing self.feature_widths_ and calculating the self.feature_indices_ property on the fly, here I store self.feature_widths_ directly
• The attribute is generated after the first transform, not after fitting. @asford implementation does fit and transform during the mapper fit. Since there is no reliable way to calculate the feature weights before applying the full transform, I prefer to leave the attribute unavailable until then. I don't know if it would be interesting to turn the attribute into a property to be able to raise a custom informative exception when some tries to access the attribute before transformation.

Thoughts?