async signup (data) {
    return await this._fetch({
      method: 'POST',
      url: '/account/register',
      body: data
    })
      .then((res) => {
        if (res.status === 200 || res.status === 201) {
          return res.json
        } else {
          throw res.json
        }
      })
      .catch((error) => {
        throw (error)
      })
  }
### signup

@param data object

{username: "barton", email: "foo@gmail.com", password: "Passw0rd!"}

@return
if ok, res.json={createdAt: "2015-12-30T15:17:05.379Z",
  objectId: "5TgExo2wBA",
  sessionToken: "r:dEgdUkcs2ydMV9Y9mt8HcBrDM"}

if error, {code: xxx, error: 'message'}