[New feature] Add logging framework

We can't always use stdio since Mruby is designed to use "embedded" to another softwares. 
It can be both of applications and bare-metal.

And some might think he could another logging destination per each mrb_state.
For example It's possible each mrb_state runs on individual threads.

Based on such requirements, I suggest to add new logging framework.

It have library default logging destinations. They are pluggable in the runtime.
It also have logging destinations per mrb_state. They also are pluggable.

Please have a look mrb_log_print() in log.c.
You'll got the rule how logging framework choice the destination of message.

This is still trial-ballon version. So I applied this framework only to test/driver.c.
I'll apply to whole code if this plan was accepted.

Please review and comments.