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
Thanks also (in alphabetical order) to @cbeams, @mikehearn, and
@tgeller, among others.
The last pre-squash commit was: c2b8d562aa107c7b68c60946cea14cdccc5159ad
