Wrap default classifier into an abstract implementation

[manthanhd]

This prepares for fully pluggable classifiers like Watson.

As of now, the following methods need to be exposed from the Classifier interface:

• getClassifications(text, callback)
• addDocument(text, topic, callback)
• train(callback)

Most of these methods are currently synchronous so first of all that will have to change.

It would be a good idea to isolate the Classifier interface into its own package.

[manthanhd]

This abstract implementation might work with something like the talkify-watson-classifier.: