DON'T MERGE: Dependson

* Course and lesson package dependencies are specified in "dependson" variable in customTests.R. This is mostly for the sake of legacy code.
* Packages plotrix, pwr, and openintro are no longer dependencies of swirl.
* Appropriate customTests.R files have been added to courses and lessons at swirldev/swirl_courses. This should not affect their operation in master. They should still be compatible with master.
* Course dependencies will eventually be specified in yaml headers. Just how to handle that is not clear at this time.