add license to opam
require that an authenticator is provided for a client.

fixes #202
style
Merge pull request #204 from mirleft/typecheck_authenticator

Typecheck the authenticator
describe approach of SNI
adjust interface of configuration
adjust implementation and checks of configuration
first select certificate (depending on SNI from client), then ciphersuite
fix compilation of lwt and examples
provide a polymorphic variant for own certificates
cleanup after type change in config.own_certificates
minor updates
example with SNI
provide interface to extract epoch from tls_lwt.Unix.t
regenerate
fix mirage examples
note about SNI
clarify startup message