Add BoxClient.ctor overload taking an IAuthRepository

+ Make BoxClient.Auth an IAuthRepository
Adds an extensibility point for apps that wish to handle authorization
and token refreshing outside of the client.
Fix endpoint URI for file deletes
Add default headers to requests

The AddDefaultHeaders() method existed but was not being called. I
assume these headers were once added; optionally specifying an
Accept-Encoding is particularly useful.