Support detection of community vs canonical version.

I've got a few internal gems used across multiple projects that depend on some of the broken behavior of the canonical version of CanCan. I'm upgrading as I can but I need to support some way of detecting which is currently in use by the host project.

This simple change just adds a CanCanCan module that can be checked for existence so we can do things like

```ruby
unless defined?(CanCanCan)
  do_some_codefu_to_fix_cancan_strong_parameters
end
```