Add DSStore and /.idea to GitIgnore file.

I'm  about to squash these, per 
1.[]How to squash commits in a GitHub pull request - Literate Programming
http://blog.steveklabnik.com/posts/2012-11-08-how-to-squash-commits-in-a-github-pull-request
2.[] ; X.How to Rebase a Pull Request · edx/edx-platform Wiki
https://github.com/edx/edx-platform/wiki/How-to-Rebase-a-Pull-Request