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 coordinate 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
Merge pull request #125 from j0k3r/button-positioning

[1.0] Attempt to fix the button positioning