support batch sending and logging & refactor tests

[springuper]

our app may flush tons of messages in a very short period, so batch processing is reasonable. Changes are:

• extends Scribe.prototype.send(category, message) to Scribe.prototype.send(category, messages), if messages is not Array it will be converted to Array
• add Logger.prototype.logMessages(level, msgs), which internally call Scribe.prototype.send(category, messages)
• refactor tests