Add implementation of ServerRequestInterface

[legionth]

This PR adds an implementation of the PSR-7 ServerRequestInterface. The new ServerRequest is based on the Request implementation of this repo.

A following PR will add a function to parse a request string into a ServerRequest.

[kirill-konshin]

This repo is a fork, so the best is to sync it with the upstream. Guzzle PSR7 has an implementation https://github.com/guzzle/psr7/blob/master/src/ServerRequest.php which is slightly different than yours.

[kirill-konshin]

I still don't see implementation of Guzzle's functions like fromGlobals and getUriFromGlobals. Please make sure that ServerRequest is a 1-to-1 mirror of Guzzle, except for 5.3 incompatible features.

[legionth]

Thank you for your response!
The functions added by Guzzle are nice, but this PR is about adding a ServerRequest to this project. The added class is still open for extensions, but adding these extensions into this PR would make it far too big to review. I would suggest these extra Features should be added in several other PRs that have a focus on it.
This should be viewed similar to the Request class which is also no 1-to-1 mirror of Guzzle.

Do you disagree?

[kirill-konshin]

Merged, but I hope you will find time to add missing things.