Post install build

As described in [#192](https://github.com/dropbox/dropbox-js/pull/192#issuecomment-52435083), currently it is hard to use a version/fork that is not published on npm since the `/lib` folder is only created in the npm prepublish hook.

The current workaround is:
 * install project dependencies
 * navigate into node_modules/dropbox
 * install dropbox devDependencies
 * execute `cake build`

This PR addresses 3 issues:
 * `cake build` on `npm install`
 * move build-related packages to non-dev-dependencies (otherwise the postinstall `cake build` fails)
 * stop using explicit paths into `node_modules` for binaries.  
    - This is required since the main project might have installed coffee-script, npm will not install it 
       twice and we need to use the coffee bin from the main project or a globally installed one.

Writing this, i think i could make this a little bit cleaner and maybe split the changes in multiple PR's
But right now, i'm out of time. So please let me know what you think about this.