Create an abstract ReceiptItem class that Expense extends.

This will allows for easy addition of an Income entity.
ShorthandReader#input() now takes a ReceiptItem to decorate.
Adding Income Entity.
Extracting the expenses list template for using with income.
Extracting a generic template out of the expenses add page.
Adding IncomeController with list and add actions.
Extracting expenses command logic into general command.
Adding tax-attack:income command.
Merge branch 'income'