Minor Additions: Payment Req Generator & 210,000 Block Halving

Two unrelated minor additions which were requested at nearly the same time.

* @mikehearn requested we add a link to @gavinandresen's PaymentRequest generator. Added to _includes/guide_payment_processing.md with link definition in _includes/references.md

* @Burrito-Bazooka requested we mention that the block reward halves every 210,000 blocks. Added to _includes/ref_block_chain.md

This PR seems non-conroversial to me, but it does include a new external link, so I'm not commiting it directly. I'll merge this after receiving one ACK or after 24 hours, whichever comes sooner.