add a skip_if option

I find this to be a very useful option that allows you to turn off rate limiting based on the setting of a particular variable (in the current request object or elsewhere).

In my case, I have a value in my settings file that I can flip on and off as I need to disable the view rate limiting (e.g. I turn it off when I'm doing loadtests against the system in some cases). But since the option takes a callable that is passed the request object, the possibilities are endless and it allows django-ratelimit to be a lot more powerful.