Create PVS.gitignore

The Prototype Verification System (PVS) http://pvs.csl.sri.com is an interactive theorem prover developed by the non-for-profit organization SRI International (http://www.sri.com). PVS is open source (under the GPL license) and it's widely used by many academic, government, and commercial organizations. Documentation about PVS can be found at http://pvs.csl.sri.com/documentation.shtml. The reasons for ignored the proposed files are the following:
* Binary files: These files are re-created by the PVS type-checker and they are specific to the OS.
* Orphaned and old proofs: The PVS prover generate this files every time that a theory is proven.
* Compiled lisp files: PVS Lisp extensions are compiled every time the system is executed. The compiled file are specific to the compiler.
* Dump files: These files are usually large, but they can easily being recreated from PVS.
* Some PVS tools such as proveit and prooflite generate log files, for debugging purposes.