ENH: add information on numbers of samples used on fit to ICA..

Since, recently, the number of samples used for fitting depends on 3 factors, i.e., n samples, the number of samples of the data, the number of samples dropped due to rejection, and the decimation). I'd like to add this information to the ICA class, especially when printing ICA objects. This will allow users to take into account the samples used for estimating the unmixing matrix when judging the goodness of the solution. Finally this PR addresses a bug related to JSON's reluctance of serialzing numpy ints, which can play a role when e.g. n_compontnes is derive from a procedure returning numpy ints.  

cc @agramfort