Implemented generator.getGuides

I implemented this for my app and thought it could be useful for others, so I'm making a pull request. Also this feature was requested for Parfait, so they could benefit from this too.

It works the same as other functions - with optional documentId defaulting to active document.

This is example of response: 

    [ { coordinate: 720, direction: 'v' },
      { coordinate: 240.9375, direction: 'v' },
      { coordinate: 1201, direction: 'v' },
      { coordinate: 0, direction: 'v' },
      { coordinate: 0, direction: 'h' },
      { coordinate: 1440, direction: 'v' },
      { coordinate: 79, direction: 'v' },
      { coordinate: 1359, direction: 'v' },
      { coordinate: 700.46875, direction: 'h' },
      { coordinate: 292, direction: 'v' },
      { coordinate: 1146, direction: 'v' },
      { coordinate: 141, direction: 'h' },
      { coordinate: 681, direction: 'h' } ]

Let me know if there are any changes needed :)

(btw: I signed Generator CLA just before this pull request)