Changing body to #wrapper to fix 3rd party style problems

Changing body to #wrapper to fix 3rd party style problems caused by too extensive rules.
Closes: #661, #596

<i>I've edited main.css manually by hand to match the updated main.less, as I couldn't figure out what less compiler and settings are you using, so I'm still waiting for your answers at <a href="https://github.com/FrozenNode/Laravel-Administrator/issues/661#issuecomment-54135725">here</a> :)</i>