Improvements

Hi, I've done a few things:

- Created a `DummyKeywordService` (and corresponding `trait KeywordService`) so we don't need the Mongo dependency, as it seems, for me, the blog and category counts are not crucial for the algorithm. I'm planning of adding a way to plug in a external source of IDF (inverse-document frequency) to be used instead of the Mongo blog/category thing.

- Created `Summary` case class to be returned by `Summarizer` instead of the actual JSON. It acts as a `Traversable[String]`to help printing/handling sentences.