Added Gruntfile.js and package.json file so we can build with Grunt

Added a Gruntfile and package.json file, removed minheader.txt and moved that text into sparkline.jquery.json so Grunt can use it when creating the minified file.

A JSHint config is also included but it's a bit restrictive and fails on the concatenated source (I use two space tabs).

The default "grunt" command (after you do a "npm install" to install the needed modules) will yield a concatenated file and minified file in the dist directory. This is useful for Bower users who want to include those files without added work.

Many thanks to Gareth for a rad library. This is my first PR, if I messed up, lemme know :thumbsup: