Check for e.defaultPrevented on 'paste'

Currently, there is a paste issue when multiple editors are initialised. Content gets pasted multiple times. It's can be seen when used with [Angular bindings](https://github.com/thijsw/angular-medium-editor), where there is currently a limitation - each DOM element needs it's own editor instance. Tracked here https://github.com/thijsw/angular-medium-editor/issues/1

This fix, solves the problem.