removing nltk dependency from the split_sentences function

I reworked the split_sentences function at line 202 so that it no longer depends on the nltk library. With this function I am able to recreate the example you have in your readme file. In the testing I've done it seems to work just fine, but there might be some edge cases I haven't discovered yet.

In my forked version, I included a version of python-goose that also has no nltk dependenceis. It was only required for one function that is not used in your code. You can check it out here https://github.com/tgallant/PyTeaser