- Began refactoring for multi-payment sales, added sales summary

~ Changes ~
- Began refactoring for multi-payment sales. Split item selection
and payment handling into two activities (as tractable multi-payment sales
will probably be a decent space hog)
- Added sales summary screen, per Jarett's request. This functions as an
intermediary screen between setup and transaction processing, displaying
information for the overall fruit stand. It serves as the point where
one starts a new transaction or ends the current session.
- Refactored existing activities to play nicely with above changes.

~ TODO ~
- Need to actually fill in logic for multi-payment sales; wanted to
get basic architecture committed first.
- Will push transaction to local database once above is completed