A Few Spelling Mistakes

These are all the spelling mistakes my spell checker caught in the
guide.  I'll be checking the ref sections separately in a few days.
Merge pull request #110 from harding/edit1

Fix a few spelling mistakes
Generate SVG And PNGs With fontname=Sans

Also remove unused PNG files and update README.
Use optipng
Shorten JBOK wallets title
Edit JBOK disclaimer to be less alarmist
Merge pull request #112 from harding/imgsans

Use fontname=Sans for images
Use "Loose-key" wallets instead of "JBOK"
Discourage JBOK wallets due to the backup hassle (not security)
Mention "Just a Bunch Of Keys" wallets
Merge pull request #113 from saivann/devjbokdisclaimer

Edit JBOK disclaimer to be less alarmist
Merge pull request #114 from saivann/devjboktitle

Shorten JBOK wallets title
Merge remote-tracking branch 'saivann/dev' into hdoverview

Conflicts:
	img/dev/en-hd-overview.svg
	img/dev/en-hd-private-parent-to-private-child.svg
	img/dev/en-hd-root-keys.svg
	img/dev/en-hd-tree.svg
Briefly Describe BIP39
Merge branch 'hdoverview' of https://github.com/harding/bitcoin.org into hdoverview