(WIP) Throttle by basic auth creds

(I'm putting this up to get eyes on it.  I'm going to test this out on some production boxes before I feel really comfortable with this.)

For our use case, we commonly use basic auth and we'd like to throttle requests by basic auth creds.

TODO:
- [ ] ~~Add rake tasks which set/clear limit params~~ I'll do this later
- [ ] ~~Make more fine grained rules so that each user can have their own, distinct limit.~~ Agreed with @bensomers that this was too big a change for this PR and could be a different PR.