Update to latest spdy version and other various fixes.

- Add client.flushConnection to allow clients to discard a connection that has
reached the max number of streams limit.
- Update the code to work with latest node-spdy (1.17.6).
- Add 'connected' event so that clients can determine when they can start sending requests.
- Fix some typos.
- Make sure opened flag is set to false in ClientSpdyConnection.prototype.closeConnection.
- Add package.json file so that spdy-client can be installed using npm (at least locally for now).