Added fix for issue 45. Asserting evidence using references will now
assert that elements along a resolvable path can only be values that
result in a valid resolution
Fix for issue #46. Importance sampling now resamples all active elements
in the universe, instead of permanent elements.
Fix for issue #48. Added a function called hasRef to ElementCollection
that returns true if the reference is resolvable on that collection
Merge pull request #50 from bruttenberg/DEV2.1

Dev2.1