Allow multiple instances of typer

Nice plugin! I found it by accident and noticed an issue with it, so I made some changes.

Fixed:
* There was an issue with using multiple typers on a page - see [this demo](http://jsfiddle.net/Mottie/7pgSB/).
* Changed height of animated cursor to match the element - used `1em` for height.
* Added a demo

Didn't fix:
* It's totally up to you how you want to name your files, but the naming convention is `jquery.myplugin.js` where `jquery` is the prefix.
* Issue with all the HTML inside of the typer element being removed.