Major bug fix, performance improvement, etc

Bug fix - .width() and .height() were being called on elements of the 'center*'
class but not necessary the correct ones. Not an issue if you only
center one element, but breaks if more than one element use the
centerVertical or centerHorizontal  classes.

Performance imporvement - Class names are stored in variables once at the beginning to be more DRY and to run
faster, as the DOM only has to be traversed once now.

Naming convention - 'midway' is prepended to the class names to make them domain specific.
'center' is removed because it's little redundant when 'midway' is in
the name. The change from camel case to dashes was to make it match CSS
notation, you can change this if you want.

Load order - I also changed the onload() call format. I think this looks cleaner, plus it's less code.

Overall, the minified file is almost half the original size, it doesn't have a pretty important bug, and it runs faster.