Refactoring 

Refactors from JW comments
- validation syntax, easier reading when alphabetically ordered
- refactor OrganizationsController#destroy 
- refactor destroy actions in ExpensesController and IncomesController
- change habtm to has_many :through for events and organizations for more easily maintainable code

Additional:
- move #create action next to #new in expenses and incomes controllers
- refactor IncomesController#create
- refactor ExpensesController#create
- extract LogExpense object
- harness the power of inverse_of

Other notes: 
- "user can destroy and event" has not been implemented yet, no destroy dependencies created. 
- incomes Controller to mirror Expense controller (and transactions)