[EPIC] Exercises Endpoint (Discussion)

(THIS EPIC IS WORK-IN-PROGRESS - IT'S NOT READY FOR DISCUSSION)

The exercises endpoint will be one of the foundational pieces of the DevLifts API, as it will touch nearly every feature. We are, after all, a fitness company.

// TODO: Add issue numbers for stories after they are created

## As a user, I:

* want to be able to retrieve a JSON of exercises for any fitness-related projects I'm working on
* want to be able to retrieve a _filtered_ JSON list of exercises based on muscle group, equipment, etc