Full Node Guide: Add Windows 8.x Instructions

[harding]

Preview: http://dg1.dtrt.org/en/full-node#windows-8x

Courtesy of @veqtrus, with assistance from @gurnec and some quick Q&A with @laanwj, this PR adds instructions to the full node guide for running Bitcoin Core on Windows 8 and 8.1. It has three commits:

1. A tiny typo fix.
2. Variablize some text in the existing Windows 7 section so it can be consistent between Win 7/8.
3. The new Windows 8.x instructions.

Note, I took @gurnec's advice in issue #825 and didn't add instructions messing around with the file permissions on bitcoin.conf in Windows.

As mentioned before, I know very little about modern Windows, so I'd heartily appreciate a through proofreading of the section by someone familar with the platform.

[veqtrus]

After downloading the file to your desktop or your Downloads folder (C:\Users\YOUR_USERNAME\Downloads), run it by double-clicking its icon. Windows will ask you to confirm that you want to run it:

You may want to change C:\Users\YOUR_USERNAME\Downloads to C:\Users\<YOUR USER NAME>\Downloads for consistency.

[harding]

@veqtrus good catch, thanks! I fixed it. Did you see any other potential problems?

[veqtrus]

This is a cosmetic detail but when not right-clicking the Command Prompt icon on the start screen (e.g. when not running it as an administrator) it looks like this.

[harding]

@veqtrus to clarify, I should use a cut-out of that image you provided to replace this image? http://dg1.dtrt.org/img/full-node/en-win8-running-cmd.png

[veqtrus]

@harding yes, no need to run as administrator.

[harding]

@veqtrus updated, thanks! Anything else I can improve?

[harding]

More feedback is always welcome, but in the absence of any major critical feedack, I will merge this around 14:00 UTC Tuesday.