Fix for accesskeys issue #839

Add the method `DomUtils.suppressAccesskeyAction` to prevent the browser focusing elements with `accesskey` set. We need this since the browser currently ignores `preventDefault` for the `accesskey` action, causing the issue in #839.
This function returns another function to restore the accesskey to relevant elements. In the case where this isn't an accesskey then there is nothing to restore. This means that, when it is called from vimium_frontend.coffee we get
```
handlerStack.push({ keyup: undefined })
```
which has the effect of adding no handlers to `handlerStack`.