Add Amount class and JsonAdaptedExpense class

[liakify]

Changelog

• Add Amount class to do self validation (expense amount must be a positive number)
• Add JsonAdaptedExpense class to support JSON serialisation of Expense
• Modified Expense class to use Amount class instead
• Add unit tests for Amount, Expense and JsonAdaptedExpense

[podocarp]

minor changes, seems good otherwise

[Aulud]

LGTM.