Add local isDefined function
Add static Keys.metaKeys collection representing all available meta keys.
Refactor Combo.clone to use Combo.fromObject
Refactor Combo.from* methods with some de-duplication, readability, and bug fixes.
Remove Combo.requires
Convert Combo.containsMetaKeys into Combo.metaKeys

Now returns the set of meta keys for the Combo, instead of a flat boolean value. The validation rules for Combo assert that all Combos have at least one meta key, so it's more useful to know _which_ meta keys the Combo uses.
Rebuild Combo.isMatch to use Combo.eq when possible, and infer intent only when necessary.
Update minified
Implement comprehensive unit tests for Key and Combo
Update documentation