Speed up Stanford tagger

[mailbackwards]

The Stanford tagger is great but big and slow. It's nearly 30mb, requires Java, and every time it's called it starts a new instance which takes a while.

Opening this issue to examine whether we can:

• Make the Stanford tagger optional so we can take it out of the repo, while keeping it very easy and user-friendly to download and integrate
• Speed it up, such as by putting it in servlet mode, see http://stackoverflow.com/questions/23322674/how-to-improve-speed-with-stanford-nlp-tagger-and-nltk