Introduce concept of configurable metric suites.

I was unsure about some style things. For example -- the little `Suite` objects, should I put them in a separate file (or files)? e.g. `lib/librato/rack/configuration/suites.rb`. I named them as a plural because the only method they expose (other than the attr reader, which could be made private) is `included?`, so it feels like a collection.

See https://github.com/librato/librato-rails/issues/71 for discussion.