Expense Domain Object and ID Set Domain Object

Created both the expense and IDSet domain object. The IDSet object was
not planned but needed because of java not allowing for Set<int> to be
used.
Setting-up Eclipse for Domain Objects and PFS

Setting up eclipse project to work on the domain objects.
Added testing for id set and expenses

Added testing for id set and expense and fixed bug in IDSet where it
would hang performing a contains call.
Update git ignore before merging.
Merge branch 'DomainObject'