Custom options and latest version of Stanford CoreNLP.

This pull request addresses 2 issues.  Supporting the setting of custom options on an annotator (see #20) and adding instructions on how to use the gem with the latest version of the Stanford CoreNLP (3.3.1) (see #25) .  The code for setting custom options was taken from https://github.com/nyarly/stanford-core-nlp.  I added some specs to test the behavior.  I also altered the specs to assume running agains the latest version of the Stanford CoreNLP.  This required altering some expectations to get the specs to pass.  Not sure if that is the correct thing to do do and if the hard code to assume 3.3.1 is right but I wanted to see some feedback.