Taskcluster-util

+++++++++++++++++++

**Features and Improvements**

- Add the credentials information.

+++++++++++++++++++

**Bugfixes**

- Fix the issue of downloading public artifact when no credentials

+++++++++++++++++++

**Features and Improvements**

- Upgrade the taskcluster library to 0.0.27, which fixed the bewit issue.
- Download artifacts by Signed URL, not API method.

+++++++++++++++++++

**Bugfixes**

- Fix the internal server error (cause by taskcluster v0.0.21).

+++++++++++++++++++

**Features and Improvements**

- Refactoring.
- Using the prograssbar package to display the download progress.
- Add hooking point 'do_after_download' after downloading.

+++++++++++++++++++

**Bugfixes**

- Fix some description error.