support BIP32 absolute path in BIP32::build_key

This might need some review since there were barely any tests for creating new derivations, so I don't really know how it has been used so far ...

I just feel that being able to specify the absolute path is more sane, specially since currently if you do `BIP32::build_key($key, "m")` you get "m/0" because "m" is unpacked as "0" ...