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

HTML preview: http://dg0.dtrt.org/en/developer-guide#hardened-keys

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.