fix module exports

[StefanoVollono]

Hi Paul, first of all thanks for your library, in my project we are using exclusively NPM as package manager and i need your library as well, i was thinking of creating my own fork and publish it on npm but actually making a pull request is a better idea :)

i've only added this if statement at the bottom of the file to export the module in case of a node envirnoment

if(typeof module !== 'undefined' && module.exports){
  module.exports = mod;
} 

if you can merge the commit and publish the package on npm it would be great! thanks again