Added fit_transform method to DataFrameMapper

[dukebody]

From #152 rebased to last master.

Attempt to solve #150 by implementing the method fit_transform for DataFrameMapper.

My implementation follows the typical approach (e.g. in sklearn's DictVectorizer) of using a private _transform( ... , do_fit) method which optionally allows for fitting. Most of the changes are therefore due to some small refactoring. 3 new test are included, and a 3 existing tests with mock objects had to be adapted but are semantically unchanged.