Refs #665: Handle double-click on Clock Out better

Redirect to dashboard with a message instead of failing
the request with a 404.

Branch: bugs/665-clockout-doubleclick
*On the invoice detail page separate billable and non-billable hours.
  * Place non-billables in a separate table so that the amount to be invoiced only includes billable hours.
  * Export should only include billable hours.
* On the Outstanding Hours page the Uninvoiced hours column should only reference billable hours.
* On the Contract detail page the left-hand table row for Hours Worked should be billable hours only.
  * Also on Contract detail page in the Project row to the right, Hours Worked should be "Billable Hours" only but add another column for Non-billable hours.
  * Clicking on the Hours Report link in this Project row should have Non-billable hours and Paid-Leave deselected, so the report renders only billable hours as well.
* On Contract list page make the Hours Worked reflect only billable hours as well.

Status: I want to add some more tests for the arithmetic, but should be
ready for some feedback on the presentation.

Branch: bug/658-non-billable
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
Refs #658: Add non-billable totals to create invoice page
Remove unneeded line that was already commented out
# Branch: bug/658-non-billable
Merge pull request #658 from caktus/bug/658-non-billable

Non-billlable time in contracts and invoices
Start 0.8.3 release notes

- and include #658
Add the rest of the 0.8.3 closed issues to the release notes.
# Branch: develop
Bump version to 0.8.3 and copyright to 2013.
Merge pull request #665 from caktus/bugs/665-clockout-doubleclick

Clicking 'Clock Out' more than once gives 404 error
Add double-click clock out bug to 0.8.3 release notes
Merge branch 'develop' of github.com:caktus/django-timepiece into develop
Add missing period in release notes.
Merge pull request #668 from caktus/develop

Changes for 0.8.3 release thus far
Merge remote-tracking branch 'origin/master' into fix/schedule