Added additional files to .gitignore

Added .DS_Store and *.Rproj files since they are both R-Studio generated
files. The .Rproj file I know has some stuff to it that may be
user-defined, but usually IDE-specific files don't go in the Git repo.
Being pedantic I know, feel free to revert whenever you want.
Removed files that were in .gitignore

After adding files to the .gitignore file, they still existed, I have
git rm-ed them to see what happens.
Merge pull request #2 from changlinli/master

Added additional files to .gitignore