Changes to Development Workflow

I've made some changes to improve the workflow during development of PIXI and its examples:

- Removed the pixi.js file in the examples -- now they all point to `pixi.dev.js` in the bin folder. These extra files were unnecessary; introducing slower build times, filesize bloat, and more work when adding new examples (now there is no need to update the build).
- A new task was created to concat with source maps. This greatly improves readability for errors (in supported browsers), allowing you to immediately see the problematic source file and line. Unfortunately we need a workaround for the `banner` option, since the plugin doesn't support this. Until then, `grunt build` has been left untouched, and `grunt build-debug` was added to generate a source-mapped `pixi.dev.js` file.
- A grunt watch task was added to rebuild (with source maps) on file changes. 

Upon changing to a more modular approach (e.g. uRequire or Browserify), source maps and watch tasks with live-reload will be much easier to integrate into the development workflow.