toolbar.js: Ensure toolbar handle cannot be dragged outside window

Previously the toolbar handle could be dragged beyond the top and bottom
of the window where it could get lost.

If the handle position outside the window border was saved, the only way
to get the handle back that I found would be to open the inspector and
manually change the handle's offset to make it visible once again.

This change restricts the handle so that it cannot be dragged above the
top or below the bottom of the window area.

Closes #634.

![django-debug-toolbar--handle-can-be-dragged-outside-window--fixed](https://cloud.githubusercontent.com/assets/342964/4160880/8ae8feda-34c5-11e4-85ad-97598c79bdc9.gif)