Remove dependency on jQuery or Zepto

I've ported the function to native JavaScript and done a bit of wrapping to let it work out if it should register as a plugin or a global function.

Feel free to refactor as desired.

Another route could be to have two versions: flowtype.jquery.js and a flowtype.native.js. If this is desired, the code for the purely native version is up at http://jsfiddle.net/Cybolic/xcLJ4/ .