Fix problem with Context not being called as a function, and make sure MathObjects are loaded.  Fixs bugzilla 3060, and Geoff's issue from pull #101.
Merge pull request #103 from dpvc/patch-PGML

PGML patch
Move ANS() call to correct place for MathObjects (so it doesn't get called for other types of answer checkers).  Resolves issue in pull request #101.
Merge pull request #104 from dpvc/patch-PGML2

Fix PGML to not call cmp() on non-MathObject answer checkers