PaymentMatrix private class implemented (keeps track of all Transaction data).
Cleaned up code; need to add more comments.
Implemented setPayers(), addPayer(), removePayer(), more helper methods in PaymentMatrix. Also started using JavaDoc to clarify potentially ambiguous behavior of those methods.
I'm not sure why it wants me to merge the UI/layout files. I attempted to have it just use the version of the files I didn't own so it just uses Manuel's changes.