Create .gitignore

Added a configuration file for Git that determines which untracked file and directory to ignore for this repository.

References for modifying this ".gitignore" file:
http://git-scm.com/docs/gitignore
https://github.com/github/gitignore
https://help.github.com/articles/ignoring-files
http://git-scm.com/book/en/Git-Basics-Recording-Changes-to-the-Repository