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
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
Fix #663 by adding addition assignment operator
Merge pull request #888 from cosmoharrigan/old-diffusion

Fix #663 by adding addition assignment operator
Revert tax behavior
Merge pull request #889 from cosmoharrigan/tax

Revert tax behavior