Added workerSentenceScore.
Modified singletons calculations to adapt it to new function structures (returning a dataframe with id's, instead of a vector of values).
Disabled insertFiltWorkers, it needs to be adapted to new table structure
* lib/filters.R
  Added Explanation filters.

* workerMetrics.
  - Added Explanation filters.
  - Storing computed metrics for spam filtering (block spammers interface).

* lib/db.R
   Added method for saving filtered workers.

* lib/simplify.R
  Created function to clean up misspellings of "N/A". To be used by the explanation filters.

* Added lib/README.md with notes about what do the filters actually do.
Merge branch 'master' of https://github.com/laroyo/watsonc