rename `.register()` method to `.set()`
change `.engines` to `.cache`
v0.5.0

Major breaking changes:

 - renames `.register()` method to `.setEngine()`
 - renames `.get()` method to `.getEngine()`
 - renames `engines` object to `cache`

These changes were made to make the method names both more consistent and easier for implementors to use without having to circumvent method naming collisions when engine-cache is inherited.