Full Node Howto: Add Windows 7, Ubuntu Server, and Other Linux

* Added instructions for Windows 7, but only for Bitcoin Core GUI. I
  added a stub for anyone who wants to write instructions for using the
  daemon on Windows.

* Added instructions for Ubuntu 14.04 LTS Server, but only for Bitcoin
  core daemon. I assume most server users run headless.

* Added instructions for Other Linux Distributions, for both GUI and
  daemon. Hopefully the instructions are general enough to apply to most
  distributions but specific enough that they actually help readers.

* Added a stub for Windows 8.1 as I don't have access to a copy, and all
  the pay-per-hour Windows VPSes I can find run some version of Windows
  Server. (I have the same problem with OS X.)

* Hid some subsections in the table of contents: I found having
  subsections named "Bitcoin Core GUI" and "...Daemon" within multiple
  sections distracting, so I hid them in the TOC.

* Added basic PGP verification instructions: I didn't try to explain PGP
  to newbies, but I did provide instructions useful to people who have
  used PGP before.  These instructions are currently displayed in the
  Windows 7 and Other Linux Distributions sections (where users download
  from Bitcoin.org).

* Made sure the end of each install section points to the Network
  Configuration section so users open port 8333.

Closes #410