Audio files upload and play

  1. Added media handling for audio and images. Image shows thumbnail as usual and audio renders player. Other attachments show download link. Added tests, moved media handling to Attachments model.
Now thumbnails are not generated for images with width smaller than 200px.

  1. Front-end got helper function `App.Post.createFromProto'. It's useful to create whole Post from proto message.

  1. Changed upload button label (to `add file')

It was my first time with Node.JS, so I'd like to hear any suggestions.