v0.4.0

CHANGES:

• Return the zone of the date (#41, @dinosaure)
• Add Content_type.to_string (#43, @dinosaure)
• Be resilient on date (accept nano-seconds) (#44, @dinosaure)
• Add a simple example to craft an email with an attachment (#47, @dinosaure)
• Drop the support of OCaml 4.08.0 (#47, @dinosaure)