Model Refactoring

The model package is removed completely. This changes the import paths of the accessory, service, and characteristic packages, which are now in the library's root directory.

Also the code for all services and characteristics is now auto-generated by importing the data from the metadata plist file in the HAPAccessoryKit.framework (see gen and cmd package). We can now import new types more easily in the future.

Existing code should migrate to the new version by replace import paths containing github.com/brutella/hc/model* with github.com/brutella/hc/*