Refactor and improve unit testing

The current implementation of the assembly plugin is that of a few really hard to manage monoliths, from which all the mojos extend. Also, the tests included in this plugin are not true unit tests, but more functional tests, in that they test the entirety of a mojo under a particular use case, as if it were a black box.

If we break the assembly plugin's abstract classes up into a reasonable set of helper/utility classes, we should be able to unit test individual pieces of the system without resorting to black-box testing of an entire mojo. This should improve the resilience of these tests when a new feature is added. Currently, it's nearly impossible to add a feature without spending a day trying to figure out why the tests are failing (a day is not an exaggeration, BTW).