Hyperlink enhancements: Editing URL, removing hyperlink (solves #35)

I've made two improvements to hyperlink editing: 
 - When selecing a link and pressing the anchor button on the bubble, the existing address is pre-filled
 - Added a way to remove a hyperlink. Currently this triggers when editing a link, clearing the address, and pressing enter.

This should resolve https://github.com/raphaelcruzeiro/jquery-notebook/issues/35

I also add `utils.selection.getContainer(selection)`, that should work cross-browser, and gets the element containing the selection.