significant terms: infrastructure for changing easily the significance h...

...euristic

This commit adds the infrastructure to allow pluging in different
measures for computing the significance of a term.
Significance measures can be provided externally by overriding

- SignificanceHeuristic
- SignificanceHeuristicBuilder
- SignificanceHeuristicParser

and registering Parser and Heuristic at the SignificantTermsHeuristicModule.

As a proof of concept, this commit also adds a second heuristic to the
already existing one (MutualInformation).
typo: "does you".  Curious why you moved away from `is_superset` as the parameter name? The new "Is_background" says nothing about the required characteristics of the background. How about "background_is_superset" ?