Attempt to fix the button positioning

`offset()` retrieves coordinates relative to the document. In our case, Medium Insert Plugin won't stand alone in most case, it will be inside an existing system.
So we need to use `position()` because it retrieves coordinates relative to the offset parent.

From the jQuery doc:

> When positioning a new element near another one and within the same containing DOM element, .position() is the more useful

Related to #116