Allow custom transport implementations

[tcrsavage]

Would be nice if we could support adding of custom transports OOTB without overriding the \ElasticSearch\Client class and static methods.

I think this could be achieved easily by using a static methods addProtocol/removeProtocol on \ElasticSearch\Client.

Similarly, we could just change self references to static in \ElasticSearch\Client::connection to allow easier class extension instead.

Thoughts?