[0.3] Replace guzzle/http with custom implementation

We have decided to no longer depend on guzzle/http. We will still be using guzzle/parser.

The reason is that guzzle/http has added a dependency on ext-curl, and I'd rather isolate react from such unexpected changes in the future. Also, we are only using a very narrow subset of what guzzle/http provides, which drags in quite a few unnecessary dependencies.

This change lifts deps on:

* guzzle/http
* guzzle/common
* guzzle/stream
* symfony/event-dispatcher

I have written minimal implementations of request and response building. Please test them extensively.