Allow header types to control their own folding.

This means that headers that are sensitive to where they're being folded
can take control, based on how long the header name is.
Update for latest rust.

to_byte -> as_byte,
Equiv doesn't exist any more, so don't use a trait on the test result.
Support decoding of RFC2047 parts within headers.
Move doc to github.
Add stability indicators, update version to 0.0.2.