Need license notice in each files

[atoato88]

Every file in this repo doesn't have license notice(Apache License, Version 2.0) at top.
It is needed for developing with this toolkit.

[atoato88]

This is non-functional topic, but important things, I think.

If this issue is reasonable for you, I'll create PR.
IMHO, this should be fix after #12.

[johnsonj]

Thanks @atoato88 - that'd be great. Agreed on sequencing.