Support for device authentication

[postlund]

Here is work-in-progress-support for device authentication required på tvOS 10.2. It should work as expected but is not complete when it comes to error handling and API. I might change those things before PR is done and do some code-cleaning as well. There's also no documentation or tests yet, I will add that when I get time.

[coveralls]

Coverage decreased (-10.6%) to 85.3% when pulling 9f5b050 on device_auth into 22512a6 on master.

[coveralls]

Coverage decreased (-0.3%) to 95.6% when pulling 0ad879d on device_auth into 22512a6 on master.

[coveralls]

Coverage decreased (-0.3%) to 95.6% when pulling 0ad879d on device_auth into 22512a6 on master.

[coveralls]

Coverage decreased (-0.3%) to 95.6% when pulling 0ad879d on device_auth into 22512a6 on master.

[coveralls]

Coverage decreased (-0.3%) to 95.6% when pulling 0ad879d on device_auth into 22512a6 on master.