Update Custom Screen docs

[mjbradford89]

https://github.com/liferay/senna.js/issues/169#issuecomment-242849732

Update custom screen docs: http://sennajs.com/docs/#screens to not rely on babel helpers.

[eduardolundgren]

Note that the current docs doesn't rely on babel, the new codebase relies on babel helpers, therefore the docs for this section using prototype will not work.