Reporting the collection on all tables

The Collection column was present only in the Reads table. The other
tables now also report this information.
Update for pymongo 2.4

The "safe" parameter is deprecated for data changing operations. It's
default value became None, instead od False. This revision will supress
pymongo's deprecation warnings.
Avoid logging explain cursors

Explain cursors, used for detailed inspection of an operation, end up
being tracked by the MongoDB debug panel. The result is that operation
showing twice in the operation tables.
Restoring compatibility with Pymongo < 2.4

Using None as the default value for the "safe" argument broke
compatibility with older versions of Pymongo. This commit reverts to
using False in those cases.
Support for Python 3
Merge branch 'master' of github.com:viotti/django-debug-toolbar-mongo