Fixes #19 - add filter validation

[logicminds]

• Previously there was not a way to validate if a filter was
  legit or not. This adds a new method that exposes the jgrep
  validation interface to facterdb.

  Additionally, it also refactors the get_facts method by adding
  an intermediary method to generate the filter string.

  I have also added a new error class for throwing a better error
  message to the user.

  Lastly, I added some method documention for all the methods I touched.

[coveralls]

Coverage remained the same at 100.0% when pulling 6816271 on nwops:filter_validation into d5cd0b9 on camptocamp:master.