Fix #8, update .gitignore and add missing file

Add the missing file static/vendor/bower_components/underscore.string/dist/underscore.string.min.js.
Before this change, the .gitignore file had 'dist' that ignores all the directory with name 'dist', but from change b22b7cfe, it looks it was intended for a particular 'dist' dir, so .gitignore was updated to only ignore that dir but not all 'dist' dir.
Merge pull request #9 from twosigma/alee/fix-8

Fix #8, update .gitignore and add missing file