Added placeholder to guide the first users.

This fix the issue #23, but the implementation is using a placeholder codemirror addon that only works if the area is not focused... so I commented the autofocus option in the editor.js file to make this work. I think is better to have the placeholder than the autofocus, but maybe you think in another way...

Test and let me know what do you think...

Abrazo.
Damián