MPSTicker: add an initializer to provide a custom tick source, and change init to use this new DI.
Fixes #1.
Explicitly expose and document init's behaviour.
Change this property to readonly in the .h and readwrite in the .m, so's I have less code to write (and one fewer custom getter to be tempted into modifying when suffering fever dreams of crazy things)
Migrate shared components into a new folder.

Using a nice little trick from http://stackoverflow.com/questions/3385187/settingup-xcode-project-to-add-new-file-in-custom-directory/21440968#21440968
Link RACTestTests with Cocoapods (to get RAC support)
Add the first test for RACTest
Remove debug log
Variable renaming
Add a test I actually don't expect to pass, but it does.