Use spaces instead of tabs

Hi there guys,

Thanks for the hubot integration, do keep up the good work on the Hall API. If you also think it's a good idea to follow some standard code-style guidelines, and bring even more people to the project, feel free to accept this PR :wink:

Solid references:
- http://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml#Code_formatting
- https://github.com/polarmobile/coffeescript-style-guide#tabs_or_spaces