Basic Uri response cache implementation

This is an effort to provide the gmaps-api-net with an customizable request cache. The cache is on the http-request level, which means it is a generic cache for all Uri request.

The Uris to be cached are not signed, so changing the Google id/key wont affect the cache.

* Adds a basic cache interface 'IUriResponseCache'
* Refactored HttpGetResponse to make use of the 'IUriResponseCache'
* The cache implementation can be set globally at 'Http.Factory.UriResponseCache', so users of this API can specify their own implementations

Note: 
There is yet no implementation of a simple 'IUriResponseCache' (I have an internal cache in my project which is used for caching)