Added gitignore file.
Added gitignore file.

Includes defaults for node, os x, windows and linux development. Feel
free to add whatever. I used http://www.gitignore.io to generate this
but you can just pull from github's too.

RobertWarrenGilmore/goll-e,twosigma/goll-e,twosigma/goll-e,RobertWarrenGilmore/goll-e