Support CheckboxSelectMultiple widget for multiple checkboxes

Added support for the CheckboxSelectMultiple widget, which displays multiple checkboxes. Based on the original templates included with django-crispy-forms for version 1.3.2, since this repo doesn't seem to support the newer 1.4.0 version. 


django-crispy-forms 1.4.0 added a new template filter that was used in checkboxselectmultiple.html, but doesn't exist in the 1.3.2 version, so I didn't include it in these changes since the existing setup.py states version <= 1.3.2