permanently remove os generated files

I noticed that [Asepsis](https://github.com/binaryage/asepsis) automatically removed some .DS_Store files. I
removed them from the repo and refined the .gitignore as suggested at:
https://help.github.com/articles/ignoring-files

Let me know if I should direct this request to another branch as you suggested in [pull #1](https://github.com/GSA-OCSIT/mygov-discovery/pull/1#issuecomment-15015022)