Plugin to show highlighted language, per Issue #1

New plugin to place a label in the top-right corner of highlighted code blocks (not inline code), displaying the current markup. Label remains in the top-right even when scrolling. 

Either the Coy or Funky theme will look a bit off with this plugin applied. See [example](http://nauzilus.net/prism/plugins/show-language/).
The Funky theme has a fully black background (rather than just the text), since the `pre > code` is now `display:block` rather than `inline`, to allow the label in the pseudo element to anchor to the wrapping `pre`. Putting the pseudo element on the wrapping `pre` works also, but the Coy theme then loses it's shadow. Since I like the Coy theme better, I'll choose not to affect that one :)