Added links to test documentation.
Like Cronet, emit error if the QUIC hint host looks wrong.
Clarified README
Introduce Java interfaces to allow multiple implementations of Cnet.
Java: partial implementation using HttpUrlConnection
Java: be more paranoid when handling exceptions.
Update README for the changes in the Java API.
Be more paranoid about object life times.
Fix race condition (GURL).

We must explicitly initialize the GURL, since its implicit
initialization is not thread safe.
JNI: refactor initilization

Now that the pool creates the fetcher, I can change the JNI logic to
avoid an upcall into Java to get the pool's native handle.
Java: more implementation for HttpUrlConnectionFetcher

- If we had a partial download, disconnect, to avoid socket reuse while
  the socket is in a bad state.  TODO: implement cancel suppression
  for small objects (but to do this correctly, we should also include
  a timeout that forces a cancellation if the small object hasn't
  completed).
- Default to following redirects (although we should add an option to
  disable this --- I added such an option for the C bindings).
- Set the HTTP method.
Java: improve API documentation.
JNI: check for NULL strings.