Add .gitattributes to enforce LF line endings.

This should allow Windows users to run the app in a virtual machine.
See this GitHub article for more details:
https://help.github.com/articles/dealing-with-line-endings/#per-reposito
ry-settings

Closes #278.
Merge pull request #291 from codeforamerica/add-gitattributes

Add .gitattributes to enforce LF line endings.