added some exercise files and fixed misprints

[snowdeer]

Hello. Thank you for your opentracing-tutorials.

I tried to exercise codes of java, and found some misprints in documents. They are simple problems, so I fixed them.

Also, I added some exercise files into lesson 01, 02 and 04. I think they can be helpful to understand your documents better.

Thank you.

[yurishkuro]

@snowdeer there have been changes since you posted this, could you please check if it's still needed, and if so rebase from master?