Merge pull request #8 from Financial-Times/demo-config-extending

prevents overrides of default config persisting to other demos
Bower install now also uses Origami Registry as a Bower registry.
Demo build enhancements:
Build service JS URLs now used.
Already-built CSS, JS and HTML files will not be built-again.
When the demo just uses the main.js or main.scss, it will use the normal build service bundles URLs.
Demo build will now update the demos property of origami.json:
if local, then it will be removed.
If build service, then it will be populated with the list of demo files that have been created.
Fixes #10
Readme now explains how origami.json is updated when demos are built.
Only install sass gem if it's not already installed or it's old. Fixes #11
Demo SASS and JS now only built for --local
Now specifies SASS gem version to install. Fixes #12
Added ability to set body class on generated demo.
Fixes for running on Windows.
Install now works on windows (fixes #14).
Bower now only installed if it's not already, or is old (fixes #13).
Made Bower version check more defensive.
Demo now adds source maps to JS and CSS, and outputs CSS in expanded style. Fixes #6.
Updated readme.
demo task now has a default config file location. Fixes #15
Added o-hoverable-on class to HTML element.
Added o--if-nojs style, and class to enable it, added to body by JS.
Removed no-js class from html element of demo template.
Demo script tag is no longer self-closing.
Added highlighting to code examples.
logging any browserify errors to console

Otherwise it's difficult to work out why your demo suddenly won't build