Goaway support added

After doing a lot of research I am confident that the current version of goaway support is done the right way.
Note that goaway frame support is optional. A spdy end point will optionally send goaway frame before it disconnects. The receiver of goaway frame is supposed to close all streams with id greater than the one present in the frame. This is the only compulsory job an end point has to do.
This PR contains
1. A processGoaway() routine in session.go
2. A test for goaway frames in spdy_test.go
3. Some readme.md updates
4. Some changes in the friendly server. The server now has an optional channel through which it sends all new session objects to the creator of the server to use. (server_conn.go)