Make lilyhdr.py deal correctly with newlines in header fields.

Substitute newlines by nulls in Scheme and translate them back in
Python.
Add the quotation marks around the header field values again.

This was messed up in the previous commit.
Beautify \0 -> \n substitution in lilyhdr.py a bit.
Make lilyhdr.py deal with "=" in header fields.
Merge pull request #4 from doughdemon/master

[lilyhdr.py] Deal with newlines in header