Added main and browser fields for npm installation

[amilajack]

@kennethcachia This allows the package to be installed with npm directly from github as a dependency. Added this because there is no npm support yet.

[shri3k]

I accidentally approved this earlier but when I tested this in my repo I got some error.
This might need more than just changes in package.json.

I'm thinking something like the following should be added if people are using CJS instead of just AMD.

  if (typeof define === 'function' && define.amd) {
    define(factory);
  } else if (typeof module === 'object' && module.exports) {
    module.exports = root.BackgroundCheck;
  } else {
    root.BackgroundCheck = factory(root);
  }

Any thoughts?