PSR-3 adapter and writer

[Xerkus]

This PR adds PSR-3 adapter for Zend\Log.
I had to add standalone adapter class as Psr logger interface can not be applied directly to Zend\Log\Logger since some methods are overlapping.

Also new Psr3 writer added to let Zend\Log write to any PSR-3 compatible loggers.

[gianarb]

I started this feature few time ago! :) Perfect 👍

[weierophinney]

@Xerkus Could you do me a favor?

• mkdir -p doc/book
• touch doc/book/psr3.md

and then document both consuming Zend\Log\Logger in a PSR-3 implementation, as well as attaching a PSR-3 implementation as a log writer to Zend\Log\Logger? @GeeH will be porting docs here soon, but this way we have the feature documented once he does, and it's not something we forget later.

BTW, LOVE the PR - very clean, very straight-forward, and a no-brainer.

[Maks3w]

PSR-3 has a compliant test suite Could you add it?

[Maks3w]

extend from Psr\Log\Test\LoggerInterfaceTest

[Xerkus]

will do
@weierophinney on a side note, this repo does not get irc notifications as it seems.

[Xerkus]

Added docs.
I will greatly appreciate if my english will be fixed there.

[weierophinney]

Merged to develop for release with 2.6.0