Dropdown toggle pip placement

This builds on top of #117 and makes the `dropdownToggle` pip position itself pointing towards the center of the initiating element.
The behaviour in actual application both on desktop and mobile:

![screen shot 2014-09-24 at 10 48 43](https://cloud.githubusercontent.com/assets/45449/4385684/ce6b42a6-43c7-11e4-88ea-ac65ae35a612.png)
![screen shot 2014-09-24 at 10 49 19](https://cloud.githubusercontent.com/assets/45449/4385685/ce6c475a-43c7-11e4-93c3-edb70bfd215e.png)

And the documentation examples:

![screen shot 2014-09-24 at 10 55 07](https://cloud.githubusercontent.com/assets/45449/4385732/908ffa02-43c8-11e4-94e6-d1b6fcd226e9.png)
![screen shot 2014-09-24 at 10 55 13](https://cloud.githubusercontent.com/assets/45449/4385733/90970bbc-43c8-11e4-8fb1-553bb9f67a0d.png)