Refs #658: Tests for nonbillable

- Add tests for the changes made in the project_hours_for_contract
  template tag. 'billable' only includes billable hours, 'nonbillable'
  only includes non-billable hours.

- Add syntax check on the template tag's new argument. If present, must
  be 'billable' or 'nonbillable'.

Branch: bug/658-non-billable