rename limit_lines_to_75_bytes to ical_format

+ \r\n replaces \n because of
http://tools.ietf.org/html/rfc5545#section-3.1
which says that it's a CRLF that separates lines.

That doesn't affect the reading, where all \r are simply ignored.
finer handling of utf-8
rename limit_lines_to_75_bytes to ical_format
make the semantics of `Glical.to_string` safer