Remove deprecated error identifier.

This whole class is probably going to die or morph, but killing off tech debt before major refactorings makes me feel good.
Remove endpoint names.

This isn't the right granularity of data about an endpoint and we never exposed this anywhere in the actual library.
Remove ResponseCallback.

Callback will be replaced with Call.
Remove Client provider indirection.

Those who desire lazy initialization can do so through a custom Client implementation which creates the delegate on first invocation. Also we can ensure OkHttp does initialization requiring i/o (e.g., disk cache) lazily.
Merge pull request #630 from square/jw/deprecated

Remove deprecated error identifier.
Merge pull request #633 from square/jw/client-provider-death

Remove Client provider indirection.
Merge pull request #632 from square/jw/kill-response-callback

Remove ResponseCallback.
Merge pull request #631 from square/jw/kill-endpoint-naes

Remove endpoint names.
Remove profiler.

This will be replaced by a generalized interceptor system.
Merge pull request #629 from square/jw/remove-profiler

Remove profiler.