Copying gitignore file from NESCent May 2014 workshop.

You don't want R's output files accidentally in your repo. This is essentially
a copy as of NESCent/2014-05-08-datacarpentry@b37cbc0bc99c79735046178c38f53d51b6730438.
Merge pull request #20 from datacarpentry/fix-gitignore

Copying gitignore file from NESCent May 2014 workshop.