md-whiteframe needs to be an attribute directive

[ThomasBurleson]

Currently .md-whiteframe-<xxx>dp used only as a class specifier.

 <md-whiteframe class="md-whiteframe-1dp"> 
    using .md-whiteframe-1dp
</md-whiteframe>

  <md-whiteframe class="md-whiteframe-2dp"> 
    using .md-whiteframe-2dp 
</md-whiteframe>

This is very confusing since all the other components and tags are Element or Attribute directives.

Much like the Layout directives, we should support the following directive notation:

<div  md-whiteframe="3"> ... </div> 

which will render as

<div  class="md-whiteframe-3dp"> ... </div> 

This is more congruent with the other components and directives in Angular Material.

Use of the md-whiteframe directive without a value should default to 4 (aka 4dp)