Merge pull request #35 from MJDSys/fix_sbcl

Fix PVS build against SBCL 1.1.16 - 1.1.18+
Add support for testing PVS against SBCL automatically using Travis CI.

Add initial support for running PVS against a few versions of sbcl using
Travis CI.  Currently tests 1.21, 1.1.18, and 1.0.58.  The current stable
versions for Mac OSX (1.1.8/1.1.6) have a bug that is fixed upstream in SBCL,
and are thus not tested.
Update the README file to point at travis, along with some more documentation.

Since travis now works, point at the build results.  Also put a short blurb
about PVS in the README file, along with a link to the homepage.
Merge pull request #36 from MJDSys/travis

Add Travis support to the PVS repository and update the README(.md) file
Fix the check-model step under SBCL.

Fix various problem with the check-model step when PVS is compiled against
SBCL:
 - Wrap the mu___append_cont so that 32bit numbers are converted to pointers.
Otherwise SBCL throws type errors.  To avoid compatibility issues, the alien
interface isn't changed.
 - Fix comparisons involving c pointers.
 - Properly check for null pointers, using an implementation proper way.  SBCL
requires a proper null check instead of just comparing the pointer to zero.