Synch docs and examples with spec - request(long)

Updating `request(int) => `request(long) in:
* JavaDocs
* `Subscription#request` in README.md, the spec itself is already up to date.

Refs #62, which changed Subscription API in 718c4d4666ccbc250bebb9cdeef7e9b5ee7b7c20

// legal stuff: this PR does not add me to CopyrightWaivers.txt, but that's in https://github.com/reactive-streams/reactive-streams/pull/92/files already, which should be good to merge soon.