Created context rules in PLN Unit test framework

Added them to PLNUnitTester file
Added rules to PLNUnitTester file
Added comments/error analysis after context rule testing

Removed juggling examples to focus on running the rule a single step
Between PN always ImplicationLink
Minor comment case fix
Fix comments (typos and explanations)
Fix constness of Router::getPortNumber and more
Merge pull request #886 from williampma/Implication

Between PN always ImplicationLink
Distinguish between deletion and purging

one removes them from the atomspace, the other kills them forever.
Merge pull request #887 from sebastianruder/context

Context rules in PLN Unit test