Add ExpenseCommand to add expenses to an Activity after creation

[liakify]

New PR from the branch with the commit history fixed. Refer to #84 for the original PR.

Changelog:

• Added missing description field for Expense class and updated tests accordingly
• Change Expense & JsonAdaptedExpense to use person IDs instead of Person objects
• Added ExpenseCommand and ExpenseCommandParser and the corresponding tests
• Added NameContainsAllKeywordsPredicate for use in ActivityCommand and ExpenseCommand (both have been updated accordingly) and the corresponding tests
• Changed Model's findPerson to findPersonAny and findPersonAll
• Implemented invite method in Activity class for use in ExpenseCommand
• Update Activity to throw PersonNotInActivityException if attempting to add an expense by a person not in the activity