Added IRestClient interface, since we need to provide alternatives for what REST/HTTP client to use. (The built-in .NET alternatives are catastrophes in terms of performance).
Added interface IRestClientFactory to be able to create IRestClient instances on the fly.
Made GoodGetModule support the injection of a IRestClientFactory.
Added class NetWebClientRestClient, implementing IRestClient and IRestClientFactory backed by the standard BCL WebClient class.
Injected the NetWebClientRestClientFactory as the IRestClientFactory to use as a default when GoodGet bootstrap.
Decoupled UpdateUsingODataFeedAuthority from the REST client implementation by having it use a given implementation if the IRestClient when fetching down data to check for updates.