refactor: removed message_type typedef
add comment explaining the non-use of pimpl
add documentation structure skel
adding doc generated files to .gitignore
refactor: moving non-static embedded_server_socket functions to headers

This is the first step to refactor embedded_server_socket into a
template.
refactor: removing non-template static methods from http socket

This change is required to refactor embedded_server_socket into a
template while the parser interface isolation is still kept.

http socket == boost::http::embedded_server_socket