Fetch tests (first batch) (#3977)

* Customizable fetch

* Some basic Fetch tests

dapps/src/apps/fetcher/mod.rs

@@ -224,7 +224,7 @@ mod tests {
 	use std::env;
 	use std::sync::Arc;
 	use util::Bytes;
-	use fetch::Client;
+	use fetch::{Fetch, Client};
 	use hash_fetch::urlhint::{URLHint, URLHintResult};
 	use parity_reactor::Remote;