Remove unecessary prepend
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
modified
fixed the bug of button positioning and the bug of image toolbar positioning
add comment for the top of $toolbar.offset()
Merge pull request #122 from j0k3r/embed-cleanup

Remove unecessary prepend
Merge pull request #125 from j0k3r/button-positioning

[1.0] Attempt to fix the button positioning
Merge pull request #127 from zh-daniel-huang/fix-button-positioning

[1.0] Fix button positioning & Fix image toolbar positioning
build dist files