Add tests for CompareMixin.compare_exclude and insert it to docs

[jedie]

https://github.com/jedie/django-reversion-compare/blob/9005d24dece15dd413bd120d6ddd54b83515b15b/reversion_compare/mixins.py#L25-L26

It's untested: https://coveralls.io/builds/14375664/source?filename=reversion_compare%2Fmixins.py

And we should add this functionality into the docs ;)

see also #84