Change to popup positioning algorithm

When using the `mentio` directive inside an element which scrolls independently from the body, the popup menu is incorrectly positioned (being shifted on the x and y axes by the scroll offset). This patch aims to solve this issue by slightly changing the positioning algorithm.
[Example before patch](http://mondora.github.io/ment.io/old/#/inside-scrollable-element).
[Example after patch](http://mondora.github.io/ment.io/#/inside-scrollable-element).

I re-run unit tests (all passed), and visually tested the change in Chrome, Firefox and Safari, and everything seems to work. But I've only tested it in the demo scenarios, so the change might break the positioning in other circumstances.

I hope the patch is useful. Thanks :-)
Paolo