Made emit_folded fold lines using existing whitespace instead of added tabs.

It now also accepts tabs as whitespace.
Use s.slice_from(_) instead of s.slice_chars(_, s.len())

The latter crashes if s contains multibyte characters, as s.len() is greater than the number of characters.
Made Rfc5322Parser::consume_char and ::peek return Option<char>.

This cuts out a lot of !::eof() calls and prevents some panics related
to reading past the end of the string.
s/rust-mime/rust-email/