Some comments, enhancements to rbmtrain, minor changes.

I added comments for some RBM parameters and a comment in tanh_opt.
I further applied some changes to rbmtrain, which I consider as elementary:
    - It is possible now to continue learning until the error doesn't change any further.
    - It is possible now to adjust the number of gibbs sampling steps.
I therefor introduced 2 new options:
    - opts.threshold to specify the threshold, how much the error has to change to abort learning. Default is 0.01.
    - opts.numsampleits to specify the number of gibbs sampling steps. Default is 1.
Note that no changes have to be done in order to keep the old behaviour.
I furthermore changed sigm and sigmrnd to use the builtin logistic_cdf function to benefit from potential optimisations of this function.