Add a ?fd argument to Lwt_io.{open_connection,with_connection,establish_server}.

The rationale is that one might want to perform sockopts to the
sockets created by those functions. They should therefore not
internally create a file descriptor for their underlying socket
without presenting a way for the user to do sockopts on them. This
patch makes it possible to provide a fd to use instead of creating a
new one, fixing this issue without changing the current interface.
Merge pull request #74 from vbmithr/master

Add a ?fd argument to Lwt_io.{open_connection,with_connection,establish_...
Implement ppx syntax extension.

Initial patch by Drup.
Implement exception match cases in ppx extension.
Documentation for ppx syntax extension.

and small grammar/spelling fixes for pa documentation.
Fix documentation and wiki generation and opam file.
Ppx documentation improvements.