HTTP client: Support for handling cookies

While I was trying to port some blocking libcURL code to ReactPHP I found there is no support for handling cookies when using the HTTP client. Inspired by other clients (especially cURL) I propose that the following features should be added:

• compliance with HTTP State Management Mechanism (expiration, send the cookie only to the corresponding domain, etc.)
• cookie support can be enabled/disabled
• cookies can be added/removed manually
• serialization/deserialization to/from a format like Netscape Cookie Jar (ideally this should be decoupled so other formats could be easily added)

As suggested by @WyriHaximus, storing the cookie jar could be done with react/cache which I find totally ok. Saving and loading to/from files could be implemented by the user or we could build something on top of ArrayCache.

After we settle the details of this, I would be happy to implement it :D

Start by reviewing RFC 6265 and the ReactPHP HTTP client implementation to resolve the proposed cookie-jar design and storage boundaries. Define how enabling, disabling, manual changes, expiration, domain matching, and serialization should work; done means the agreed behavior is implemented and covered by tests.