getBlob(sha, cb) {
      return this._request('GET', `/repos/${this.__fullname}/git/blobs/${sha}`, null, cb, 'raw');
   }
Get a raw blob from the repository
@see https://developer.github.com/v3/git/blobs/#get-a-blob
@param {string} sha - the sha of the blob to fetch
@param {Requestable.callback} cb - will receive the blob from the API
@return {Promise} - the promise for the http request