A configure option now allows the user to avoid installing the gsettings-data-convert file
(shotwell.convert).  This is useful because the file must be installed in
/usr/share/GConf/gsettings (and not /usr/local) for the program to locate it, which is a
problem for folks who want to install as a user and not root.

Also cleaned up the configure script help, which was still reflecting options no longer
available.