Refactor Adapter and RestAdapter

This PR provides the following changes:
* Adapter no longer requires an IContext instance
* RestAdapter offers a constructor without IContext
* IContext itself given an attribute to identify User-Agent

Rationale:
From looking at the sources of both the iOS and Android SDKs, it is apparent that IContext fills no other function than nailing down a specific User-Agent. Due to the Android-orientation of the interface, it becomes virtually meaningless in the cross-platform paradigm that Xamarin works on.

This PR removes IContext completely from Adapter, since this fundamental class needs to be as generic as possible. IContext remains in RestAdapter for now, but an extra constructor is offered in order to allow implementations without using IContext. In this instantiation strategy, the RestAdapter simply assigns "loopback-xamarin/1.0" as User-Agent.