modernize INSTALL_PATH

generally for OS X 10.5 and above using @rpath for the install path
offers the most flexability in where the framework may reside.
drag image not visible when dragging last tab

when dragging the last tab from a window the drag image is not visible
thus the tab bar button and mini or alpha window are not seen while
dragging.
Merge pull request #31 from nivekkagicom/master

fix for dragging last tab from a window and change to use @rpath