add fetcher to fetch JS dependencies

This small class adds a feature to easily fetch JavaScript dependencies from their project pages, e.g.:

     bin/catapult fetch jquery

would 'fetch' jQuery and it could easily piped into your new project with '> assets/libs/...'

Currently supported fetches are:
 
      'jquery'       => 'http://code.jquery.com/jquery-1.9.1.js',
      'underscore'   => 'http://backbonejs.org/backbone.js',
      'backbone'     => 'http://underscorejs.org/underscore.js',
      'handlebars'   => 'https://raw.github.com/wycats/handlebars.js/1.0.0-rc.3/dist/handlebars.js',
      'marionette'   => 'http://marionettejs.com/downloads/backbone.marionette.js'


I can imagine some variations of the theme to help to setup a fresh project from scratch (e.g. using a YAML, automatically saving the files in the corresponding folders, etc.)

What do you think?