Attempt to address https://github.com/driftyco/ionic/issues/2682.

Adds an optional hide-when-closed attribute to the ion-side-menu
directive. If this attribute is set to "true" the side menu will be
hidden from the document layout whenever it is closed.

This was branched from the v1.0.0.-beta.13 tagged branch

It's my first pull request, so apologies for perhaps not quite adhering to the guidelines yet....