hook up OCaml-TLS client (and prefer over lwt-ssl (OpenSSL))
require tls and tls.lwt
move TLS/SSL selection into start:
if both lwt.ssl and tls.lwt are available:
select based on CONDUIT_TLS environment variable (defaults to lwt.ssl)
if only one library is present, use it
if none, No_tls
default to OpenSSL / lwt.ssl
add OCaml-TLS server code
add interface file for tls.lwt
rename `OpenSSL to `TLS
support server for OpenSSL (lwt.ssl) and OCaml-TLS (tls.lwt) stacks
configurable via compiled dependencies, and CONDUIT_TLS environment variable
defaults to OpenSSL
update copyright
Merge branch 'ocaml-tls' of git://github.com/hannesm/ocaml-conduit into hannesm-ocaml-tls
Rationalise the client TLS types to add in explicit selection of
individual libraries (OpenSSL OCaml_native) in addition to the
default selector logic
Make the `Conduit_lwt_unix.client` type follow the same convention as `server`

This turns the client type into a set of polymorphic variants that name record
fields.  It is a more heavyweight style, but lets configuration fragments be
structural (which records arent).
Add explicit TLS selectors for the TLS server side configuration
Sync CHANGES and bump version to 0.7.0
Document the CONDUIT_TLS env variable
Add more debugging output for when CONDUIT_DEBUG is set on Unix