Release 0.6.0

• PR #5 from @hummingly: Adds ZapfDingbats encoding
• Fix Encoding::encode_string. The encoded bytes b'\', b'(', and b')' must be escaped properly, not the unencoded characters '\', '(', and ')'.
• Some documentation improvements.
• Remove some explict lifetimes.
• Avoid some cloning. Or at least delay it slightly, cloning in caller rather than callee.
• Code-style changes. Mainly use x? instead of try!(x) and follow rustfmt updates.
• Testing now also done on windows, by appveyour.