[scikit-learn] partial_fit implementation for IsolationForest

[donkey-hotei at cryptolab.net]

hello scikit-learn devs,

After following the work on IsolationForest so far and testing on a 
real-world problem here we've found this model to be very promising for 
anomaly detection. However, at present, IsolationForest only fits data 
in batch even while it may be well suited to incremental on-line 
learning since one could subsample recent history and older estimators 
can be dropped progressively.

I'd like to contribute this feature, but being new to ML and 
scikit-learn I'm curious how I should start making a quick & dirty 
version to see how this may work. Are there other good examples where 
one could see the difference between .fit and .partial_fit in other 
models?

thanks
isaak y.