Add .gitignore rule to ignore temporary .swp backup files created by vim. (google#859)

- Addresses : google#858

  - Rule `*.swp` is added to `.gitignore` to ensure that the vim temporary
    `.swp` backup files are ignored and they don't pollute the results of
    `git status -u`.

Author: aerosayan

.gitignore

@@ -8,6 +8,7 @@
 !/cmake/*.cmake
 !/test/AssemblyTests.cmake
 *~
+*.swp
 *.pyc
 __pycache__