Add one-hot scheme aka one-of-C encoding
Apply requested changes

* pass range of categories as argument
* output can be sparse
Indicator matrix for multiple features
Add `indicators` for generic categories
Remove unused code
Clean up `indicators`

* sparse is a keyword argument
* remove special handling for numerical types
Merge pull request #22 from AlexanderFabisch/one_of_c_encoding

Add one-hot scheme / one-of-C encoding