Sentiment NLU middleware

[timkinnane]

Not sure yet if sentiment should be handled as part of NLU adapter by default, but some services might not provide it and some sentiment services won't provide intent/entities either.

First draft demonstrating sentiment should probably be on NLU middleware, implementing one or multiple of the following:

https://medium.com/@sifium/top-five-emotional-sentiment-analysis-apis-116cd8d42055

This is probably more for a demo than a core feature.

[timkinnane]

https://market.mashape.com/japerk/text-processing

[timkinnane]

This is resolved now by recent updates which settled on a universal NLU result that can contain sentiment or any other data. Is implemented by the Watson Tone adapter and will be illustrated in #52.