Billable Expenses shouldn't be added to actual project cost

[#1]

Problem: Currently, if you add an expense to a project and you check the billable check box, the expense is added to the actual cost of the project. Theoretically, this shouldn't be the case because an expense that is marked as billable means that we're expecting to recoup those $$.

Proposed solution: Billable expenses shouldn't be factored into the actual cost of the project.