Overhauled Instrumentation loading API

Strumbar's instrumentation API has been revamped and is not backwards compatible.  Instead of the
'eager-loading' configuration, Strumbar.configure now has an instruments interface that allows
loading and unloading of objects for instrumentation.

Strumbar.configure do |config|
  config.instruments.use Strumbar::Instrumentation::Redis
  config.instruments.delete Strumbar::Instrumentation::Redis
end

This interface accepts any object/class/module that responds to 'load', and will raise an exception
if you try otherwise.

Strumbar::Configuration#instrumentation is now removed.

Finally, Strumbar's default instrumentation will no longer autoload by default.  Those who wish to
use the instrumentation provided by Strumbar will have to add them via the new
`config.instruments.use` interface.