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