Move to Dagger 2

[codefromthecrypt]

Dagger 1.x is not likely to continue. Dagger 2 is.

I'm interested enough to help port everything here (and maybe fix some bugs etc along the way), plus revamp upstream feign accordingly.

Please ping this issue if you feel strongly that feign should not move to Dagger 2.

[codefromthecrypt]

The Dagger 2 approach is significantly incompatible with Dagger 1, particularly as multiple components with singleton scopes are unsupported, as is overrides. It is probably better to remove dagger so that callers can choose dagger 1, dagger 2, spring, guice, etc.