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.