Cleanup the Gruntfile

[mtraynham]

Hey Gordon,

This is basically round two of #844. Some things that have been changed here:

• Upgrade the npm dependencies to the latest-minor versions. Seems I didn't catch the minor numbers previously.
• Add load-grunt-tasks and time-grunt dependencies. These are small grunt utilities, one of which removes redundancy in the Gruntfile, the other logging the time per task.
• Remove the grunt-sed task. grunt-contrib-concat has a process flag that allows us to use Grunt templates in the source files (e.g <%= conf.banner %>)
• Take advantage of the banner property on concat and uglify tasks. No need to put it in the source code, so removed from src/banner.js.
• In the Gruntfile, use config templates where necessary. Just convenience if you decide to move files.
• Fix the sourceMap property of the uglify task. Should be a boolean.
• Removes the difflib.js file from the stock-regression suite. This can be pulled from npm.

In the future, I'd prefer moving away from putting inline script tasks/multiTasks in the Gruntfile. There's minimal documentation around what they do, and it looks a bit convoluted.