Add Travis support to the PVS repository and update the README(.md) file

This series adds a .travis.yml file, which enables building on Travis CI.  Once the patch set is merged, just enable support on travis-ci (https://travis-ci.org/) for the project (I recommend enabling the "Build only if .travis.yml is present" option).  Then all pushes/PR's will be checked by Travis to ensure they build against SBCL.  Currently it tests 1.0.58, 1.1.18, and 1.2.1.  1.1.8 (the version from Mac OSX) won't build PVS due to SBCL bugs, which have since been fixed.  I don't think it is worthwhile chasing down the behaviour triggering them in PVS.

I've also updated the README file (and moved it to README.md) to include a button showing the current Travis status, and applied some formatting to make it easier on the eyes.  I copied the description from the PVS home page.

Note I also learned Travis CI now includes easy support for deploying build products to various services, from something as simple custom script to an FTP site to PaaS providers or Amazon S3.  If you want binaries automatically pushed, let me know and I'll investigate.