Kullback-Leibler divergence as a parameter of TSNE

As in [1], I think it may be useful for the user to have access to the variable 'error' (Kullback-Leibler divergence, AFAIC). In fact one can choose between many t-SNE embedding on the same dataset by looking at the error [2].

[1] http://datascience.stackexchange.com/questions/762/t-sne-python-implementation-kullback-leibler-divergence/
[2] http://homepage.tudelft.nl/19j49/t-SNE.html
add a trailing slash -> `error_`