createBlob(content, cb) {
      let postBody = this._getContentObject(content);

      log('sending content', postBody);
      return this._request('POST', `/repos/${this.__fullname}/git/blobs`, postBody, cb);
   }
Create a blob
@see https://developer.github.com/v3/git/blobs/#create-a-blob
@param {(string|Buffer|Blob)} content - the content to add to the repository
@param {Requestable.callback} cb - will receive the details of the created blob
@return {Promise} - the promise for the http request