swc-installation-test-2.py: Add PathCommandDependency

OS X doesn't believe in $PATH ;).  Work around this heresy by simply
checking for the existence of a characteristic file or directory
(e.g. /Applications/SomeApp.app).  I use the new class to add 'xcode',
'sublime-text', 'textmate', and 'textwrangler' installations.

TODO: determine the canonical Xcode installation directory