Improve unicode label support

Currently we could only get UTF-7 encoded label from the public API. This pull request use the implemented `utils.py` to improve support for unicode label.

This modified is compatible with old usage, and added more such as calling like `gmail_account.label(u"中文标签")`. A new optional argument has been added to `Gmail.labels` method, `gmail_account.labels(require_unicode=True)` will decode all UTF-7 string to human-readable label.

Please review, thanks. :smile: