Consolidate plugin imgjpeg into core.

Move jpeg functionality from plugins-main/src/jpeg/imgjpeg.c to src/jpeg.c
Add the option controlling jpeg compression quality in core.xml.in
Add -ljpeg to src/Makefile.am

NOTE: Plugin imgjpeg was imported from
git://anongit.compiz.org/fusion/plugins-main
at commit 611295a4d4a9dce76316056c79741c387979e730