Implemented basic GET requests
Implemented request headers and body.
Don't cache non-GET requests and non-200 responses

Modified caching-request so that it does not cache responses to requests
that are not GET.

Added a test to check that non-200 responses are not cached.
Implemented maxAge and etag support.

A cached response is returned only if it's younger than maxAge seconds.

Requests include etag from the last cached response.
Started TODO
Implemented maxStale.
Return cached version on transport error.

Modified CachingRequest to return a cached response in case of a transport
error.
Enabled strict mode.