Fix a few JS warnings that pop into the console
Fix the least controversial jshint warnings

To install jshint, simply `npm install jshint` from the root
directory of the project. Then run like it like this:

    node_modules/.bin/jshint data/*.js
Remove unnecessary execute bits on files

These files should not be executable
Add a note about running jshint in the CONTRIBUTING file
Merge pull request #560 from fmarier/fix_js_warnings

Fix js warnings