Helper Class for Fluid Vertical Alignment.

Adding helper classes for vertically aligning a child inside it's container without needing to define fixed heights. I find this useful for rapid prototyping fluid sites. This is supported in IE8+ and all other browsers that support pseudo-elements. I apologize the fixed width of 300px on the child element shouldn't be there.

Reference: 
http://css-tricks.com/centering-in-the-unknown/