Add Ruby equivalent of fann_train

Hi.

I've been hunting a bug in my code and I needed to call `fann_train` from Ruby.
I didn't find the function in your bindings, so I added it.

I noticed that `ext/ruby_fann/Makefile` seems to be generated with configuration specific
to your machine. Should I include the changes my build of ruby-fann made?

By the way, why is this Makefile under source control? I think it would be a good idea
to ignore generated files. (Same goes for `ext/ruby_fann/*.o`.)