PermissionsRequiredMixin: Add helper functions for overiding
Subclass PermissionRequiredMixin in MultiPermsRequiredMixin

Allows for better code re-use and overriding of default behavior.
Update docstrings and comments for clarity.
Update doc string
Merge branch 'master' into pr/67
Merge branch 'master' into pr/67
changelog
docs update
merge conflict.
Merge pull request #122 from brack3t/fix/permission_required_custom_perms

Fix/permission required custom perms