Mention Bitcoin's RPC Hash Byte Order

* Add a short subsection about the different byte orders used with
  hashes in Bitcoin Core and other software.

* Re-word some text in other sections to mention the byte order
  differences

This commit created based on comments from @SergioDemianLerner (thanks!)
Describe Soft And Hard Forks

Based on a suggestion made by @petertodd to the -devel mailing list and
the discussion in that thread by him and other participants.

* We've been using the term "consensus", but this commit introduces a
  formal definition for it and "consensus rules" as part of the block
  chain introduction.

* Describe that consensus rules may change and may happen when they
  do: hard or soft forks.

* Describe how full nodes can detect hard or soft forks, then describe
  how SPV clients can detect hard and soft forks using the more limited
  information available to them.
Link To Detecting Forks From Verifying Payment Section (thanks @saivann!)
Merge branches 'consensus-forking' (pull #615) and 'byte-order' (pull #583)
Reorder Links (No URL Or Text Changes) & Fix Tiny Typo

* Sorted the _includes/references.md file as it became unclear where
  links should be added.

* Fixed accidentally lowercased "merkle" at start of sentence.