Switching instrument_file? to be in Instrumentation class

This was done to solve the RequireJS issue (https://github.com/modeset/teaspoon/pull/240), would like to hear your thoughts about this.  Some things:

* The reason why this change works with RequireJS is because the decision to instrument is now made in the Sprockets hook, and all RequireJS requests go through Sprockets.
* I wasn't sure how to grab the current config so I grabbed the default config with `Teaspoon.configuration.suite_configs['default'][:instance]`.  I can help write specs once this part is figured out.
* I was thinking about completely forgoing the addition of `instrument=1`, but wasn't sure which way to go.  Either way works with RequireJS, as you can add a `instrument=1` to the end of every RequireJS request through the `urlArgs` config.