Make Clearer The Benefits Of Hardened Keys And The Absence Of A Master PubKey

As suggested by @gmaxwell (thanks!), I tried to make clearer the benefit
of hardened keys:

* Described hardened keys as a solution in the first sentence of the
  Hardened Keys subsection.

* Reordered the text so that the problem is described before the
  solution, making the presence of a solution clearer.

* Added a prefatory sentence to the description of the two key
  derivation formulas again describing the hardened formula as a
  solution.

As suggested by @vbuterin (thanks!), I added a paragraph describing that
HD wallets don't use normal derivation on the master key so they don't
have an effective master public key. (See end of the diff.)

This is a fairly large diff because of the reordering, but no new
clauses were added besides those described above.