Remove a version of TSNE::computeGaussianPerplexity that appears unused

There are three definitions of computeGaussianPerplexity, but only two
calls. The uncalled version appears to be this one.
Remove exact tsne option.

It isn't used. Why keep it?
Remove commented out main() function
Finish exact t-SNE value removal (oops)
Add option of cosine distance

It could be that C++ is a TERRIBLE language, or it could be that I just
don't know it well, but there seems to be no simple way of sharing the
tree building code between the cosine_distance and euclidean_distance
versions.

Relatively untested: I know it does *something*, as in lays the MNIST
dataset out in a peculiar spiral, but I don't know if that is the right
thing.
delete trailing whitespace.
gitignore backup~ files