mdDialog: clickOutsideToClose responds to mouseUp even if mouseDown was inside the dialog

[BodrickLight]

Steps to reproduce:

1. Open a dialog with clickOutsideToClose set to true.
2. Hold mouse button down anywhere inside the dialog.
3. Move the mouse outside the dialog while still holding the mouse button down.
4. Release the mouse button.
5. The dialog closes.

This can be annoying with slider controls inside the dialog - if you click and drag the slider outside the boundary of the dialog, the dialog will close. It seems like the dialog should only close if mouseDown and mouseUp are both outside the dialog.

http://codepen.io/anon/pen/yNRBBo