Added books/projects/hybrid-systems/.

The new directory books/projects/hybrid-systems/ is an ACL2(r)
specification/verification project by Shant Harutuntian in support
of his 2007 Ph.D. dissertation, with a few recent updates (because
of ACL2 changes) made by Cuong Chau.  These files are being added
with Shant's permission (and also Cuong's).