Javascript metamodule

This adds a new modules file with a target meta-gtk-osx-javascript
which builds GJS and its dependencies (mozjs17, nspr).

This target exists in modulesets-stable (GJS 1.38.1) and modulesets
(gnome-3-10 branch), but not yet in modulesets-unstable; I haven't
gotten mozjs24 to build yet. In both cases, mozjs17 and nspr are built
from tarballs.
js24 and Javascript for modulesets-unstable

Master-branch GJS requires js24 instead of js17. This adds js24 to
all three modulesets (since it builds from a tarball anyway) and also
adds a meta-gtk-osx-javascript target for modulesets-unstable, since
we can now build GJS from master.
Merge pull request #24 from ptomato/javascript-metamodule

Javascript metamodule