Tidy README (better text formatting & structure)

Fix #148
Change to "Install"
Link to releases tab
Add bower install target
Tweak Bower and Grunt install text
Quote tweaks
Remove sections as instructed by @potch

https://github.com/mozilla/brick/commit/feaf83e29bdd591d800043180d9833d26f7d1cfa#commitcomment-5746372
Merge pull request #149 from mozilla/readme-copywriting

Tidy README (better text formatting & structure)