Added a class to body which enables/disables layout transitions

Added a class to body which enables/disables layout transitions

This allows people to disable animation of CSS properties temporarily
if needed by removing the vector-animateLayout class from the body.

// Disabled CSS transitions on properties customized by the hd layout
$( 'body' ).removeClass( 'vector-animateLayout' );
// Change something that would normally be animated
$( '#content' ).css( 'margin-left', 0 );
// Re-enables transitions
$( 'body' ).addClass( 'vector-animateLayout' );

Change-Id: I6d164e45c1e31d4f83e0fa390011c5d182eb69c3