Minor changes to make sigma publishable to NPM

Fixing #186. Here are some more details about the commit itself:
 - Updated conrad.js
 - Added a dirty script to fix exports: sigma.export.js
 - Fixed last references to window
 - Added the npmPrePublish Grunt task, and linked it to `npm prepublish`
 - Added .npmignore rules and fixed some in .gitignore
 - Updated package.json, changing the name from "sigma-js" to "sigma"