Refs #655: Don't create two invoices on double-click

This uses a Django 1.4 feature and so should not be merged until we've
dropped Django 1.3 support.

Branch: bug/655-double-click-invoices
Refs #655: Don't create two invoices on double-click

This uses a Django 1.4 feature and so should not be merged until we've
dropped Django 1.3 support.

Branch: bug/655-double-click-invoices
Merge branch 'bug/655-double-click-invoices' of github.com:caktus/django-timepiece into bug/655-double-click-invoices
Merge remote-tracking branch 'origin/develop' into bug/655-double-click-invoices

Conflicts:
	timepiece/views.py
Merge pull request #655 from caktus/bug/655-double-click-invoices

Create Invoice creates two invoices if double clicked