Replace Map based iter with own implementation.

This should hide the horrid std::iter::Map return type.
Add parser for reading RFC822 dates.
Add FromHeader impls for DateTime.
Make boundary at MimeMessage::new* functions.

This should mean that we don't need to have as_string() as mutable,
fixing #4.
Add function to MimeMessage to support updating based on state.
Add semicolons after macros that expand to items.
Merge branch 'bug/message_mut_as_string' into develop
Final multipart boundary should have a "--" at the end.
Update lazy_static requirement because of macro change.
Bump version to 0.0.5 for cargo upload with datetime impl.