implement HttpTransport

i took a first stab at #6, doc-blocks and especially tests are yet missing but will be completed - i'd be first interested in some feedback from @bzikarsky? :) 

Is that - at least a bit - like you'd thought it should? `HttpTransport` is mainly based on `UdpTransport`.. i'm open to suggestions, if we should make it more flexible at certain points?
Hm, I'm not 100% sure about the compatibility of Graylog2 with HTTP/1.1 and keep-alive connections. To keep the TCP-connect-overhead as low as possible we should aim for a keep-alive connection