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
Fix accessibility of PSMTabBarControl

* it's now an AXTabGroup - thanks to AXTabs the position of the tab
  and total tab count are announced
* AXChildren shows only visible tabs, and all buttons (like overflow)
* AXValue + AXValueChanged notification enable announcing new tab name
  when switching between them with Cmd-Shift-[] (for unknown reason
  this does not work quite yet)
Add accessibility description to the overflow tab bar button
Fix accessibility of PSMTabBarCell

* implemented visual bounds - yields proper VoiceOver cursor bounds,
  proper order of presentation in sequentional navigation (VO-left and
  right arrows), and also enables touch exploration
* implemented AXValue - so that the selected tab can announce itself
  as such (AXFocused is for elements having keyboard focus, i.e. being
  first responder, which is something quite different than being
  selected from a list of other options)
Add accessibility description of the add tab button
Merge pull request #32 from dusek/accessibility

Fix accessibility
Correct the desired width for Adium's tabs from double counting the icon and close button.
AdiumTabStyle: Scale the icon for the maximum width allowed.
-convertRectToScreen is not available on 10.6.