[FEATURE] Add HTTP-version and start-line support to Http\Message
This adds support for the HTTP-version to HTTP Messages like
Request and Response; i.e. it is stored and accessible.
As a consequence, convenient getter methods for the so-called
"Start-Line" (RFC 2616, section 4 HTTP Message) is added,
being either the "Request-Line" or the "Status-Line" depending
on the Message implementation (being a Request or a Response).
See RFC 2616, sections 5.1 and 6.1 accordingly.
Besides, it makes Http\Message an abstract class because
an HTTP message must be a Request or a Response and cannot
be a Message itself.