adds Instrumenters to library

- Instrumenters::Noop for when an instrumenter is not specified in config
- instrumenters::Memory for use in specs

refs #17
adds Instrumenter::Noop as default instrumenter in config

refs #17
fleshes out instrumenter support at the DSL level

- instrumenter method only used for setting config[:instrumenter]
- adds instrument method that passes arguments thru to the configured
  instrumenter so that you can call `hyperdrive.instrument` from any
  where in the code (internally or by users of our library)

refs #17
adds sugar for calling the configured instrumenter

adds ability to call `instrument` instead of `hyperdrive.instrument` in
callbacks and request handlers

refs #17