Overwrite BaseClient's getItemURL with our own implementation

TODO: getItemURL still needs to be implemented

@param {object} rs - RemoteStorage instance

@private
function hookGetItemURL (rs) {
  if (rs._origBaseClientGetItemURL) { return; }
  rs._origBaseClientGetItemURL = BaseClient.prototype.getItemURL;
  BaseClient.prototype.getItemURL = function (/*path*/) {
    throw new Error('getItemURL is not implemented for Dropbox yet');
  };
}