Fixed launching of cmd-key-happy via launchd on Mavericks

Added an additional step for the installation on Mavericks.  The
makefile rule 'install-security-exception-for-cmd-key-happy' now updates
the database which governs which applications can use the accessibility
APIs.

On Mavericks, as noted in the install file, the installation procedure
is now:

  $ make install
  $ make install-rcfile   # only if you have no ~/.cmd-key-happy.lua
  $ make install-security-exception-for-cmd-key-happy
  $ make install-plist

Closes github issue #12.