Add "Hide Label" option for full-width Archetypes

Adds a Prevalue Option to hide the Umbraco property label, allowing the Archetype to span the full width of the page:

![2014-03-14_22-35-23](https://f.cloud.github.com/assets/1396376/2427624/59a9cbf4-abfb-11e3-88ac-72267fa58c5e.png)