refine & document order of uninstall keys

Proposed order - :script, :launchctl, :quit, :kext, :pkgutil, :files.  Quit
a process before attempting to unload an assocated kext, unload kext before
attempting to delete the associated file, etc.  Arguably :script fits
loically with :files near the end of the list.  However, we also have
:after_uninstall which implicitly fires immediately after :files.
Therefore, running :script early provides greater functionality.
Merge pull request #2024 from rolandwalker/uninstall_order

refine & document order of uninstall keys