[MRG] Remove compute_importances

This basically solves #1447. 

Feature importances are now computed upon access to the feature_importances_ attribute. 

As a result, the compute_importances parameter is no longer required. Importance scores can be computed on the fly (without rebuilding the entire model because one forgot to set compute_importances=True). 

Just like in the GBRT module, it relies on a Python property.