Feature/relevance scores

Opening this PR more as a review than an actual PR to be merged.

## Meaningful Changes

- So in this pr we introduce `Relevance Scores` which is the relevance of a topic to the given text as a whole. Basically is a number between 0.0 and 1.0.

- `RelevanceScores` are returned per each topic spotted in the text.

- the relevance of a topic can be found in the field `relevanceScore`

- If the property `relevance_scoring` is set to: `default` in `models.properties` then the relevance scores will be calculate. If it is not defined then relevance scores are not calculated and a default value of `0.1` is returned in the field `relevanceScore`

## More details

- `BaseRelevanceScore.scala` contains the class calculating the score discussed in the gist below
- `RelevanceScorer.scala` is more of a glue/paste class that I did to sort of follow what I thought was the convention in spotlight. 

- [About relevance score calculation](https://gist.github.com/dav009/53834138ffec2d878acf)

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@thiago any thoughts ?