New Rule: Enforce property shorthand

[DanPurdy]

For more info see #15

This rule should enforce the use of shorthand properties where possible such as the following

.selector {
  background-color: #000;
  background-image: url(images/bg.gif);
  background-repeat: no-repeat;
  background-position: top right;
}

Should actually be

.selector {
  background: #000 url(images/bg.gif) no-repeat top right;
}