wrap fits_hdr2str()

One more function.

This is important for using a WCS library, where the WCS is initialized from a header string.

I realized that `bytestring` creates a copy, meaning that the entire header string will be copied, seemingly unnecessarily.  I don't know how to avoid this though... seems that it would require telling julia to reinterpret the `Ptr{Uint8}` as an `ASCIIString` and also to take ownership of it. It wasn't quite clear to me from the C interface docs if this is possible (or for that matter, recommended).