Support synchronize printing

[sanak]

Supports #33.

Changes proposed in this pull request:

• [x] Support sync print downloading.
• [x] Configurable async/sync print setting.
• [x] Fix and add tests.

@gtt-project/maintainer

[sanak]

@mopinfish ありがとうございます！ 1.1-stable ブランチまでマージします。