Convert storage.headers to storage-specific parameters

[markmhendrickson]

Currently, the logic for the storeFile method in the item controller hard-codes Dropbox headers:

headers: function(path, userStorageAuth) {
    return {
      'Authorization': 'Bearer ' + userStorageAuth.storageToken,
      'Content-Type': 'application/octet-stream',
      'Dropbox-API-Arg': JSON.stringify({
        autorename: false,
        mode: 'add',
        mute: true,
        path: path
      })
    };
  },

Extract these headers from the code and integrate them via storage-object-specific parameters that dictate what headers to use per storage.