Added support for PSR18

[Nyholm]

I know this PR maybe premature for you and we may not be ready to drop PHP5 support. I'm soon about to open the official vote for PSR-18. And I wanted to show how it will effect Guzzle.

See the specification: https://github.com/php-fig/fig-standards/blob/master/proposed/http-client/http-client.md
Read the blog post: https://medium.com/php-fig/the-http-client-psr-9c2535132980

[jeremeamia]

Seems pretty straightforward.

[Nyholm]

The voting for the working group is open. @jeremeamia, add your vote here: https://groups.google.com/forum/#!topic/php-fig/BsroPEZcr_M

[gmponos]

Maybe you could have a look on this #2163. If we could merge this PR you can have this exception extend that one.

[boesing]

How about a satellite project for guzzle to support PSR-18 instead of directly integrating it here?
Something like guzzlehttp/guzzle-psr18 with just a decorator inside.

Could work like the decorators for zend-cache which provide either PSR-6 or PSR-16 support.
https://github.com/zendframework/zend-cache/tree/master/src/Psr/SimpleCache
https://github.com/zendframework/zend-cache/blob/master/src/Psr/SimpleCache/SimpleCacheDecorator.php

This wont force guzzle to make a major release to drop PHP 5 support and there will be an official solution for guzzle to be used with PSR-18.

[Nyholm]

This is merged in master.