Modified for testability

I've started to use this plugin, but would like to potentially add a few features for my own use.  One feature would be the ability to set servers in a global setting.  It's kinda a pain to have to update each individual project if the password for the user account has changed.

But, before working on that, I wanted to make sure the current plugin is tested (since there wasn't a single test case yet).  I broke up the StashNotifier into various components that are each individually testable.  The functionality is currently still the same.  For the most part, the code is even the same, just moved around a little to make it easier to test.

It was my first time using Mockito (chose to use it since it's "the Jenkins way"), so if something looks a little off, just let me know.

Anywho... let me know what you think!