Checksum and encoding cleanup

changed Base64 encoding of binary data
to hexadecimal Big endian encoding like the checksums are
there are more than one Base64 implementations available, that means
if one gets the idea to transmit key data in binary and reasseble it
to base64 he must know wich Base64 to get the checksum back.

all data stored as hex Big endian now, so you can have a binary
representation of all the data. The public key base64 version is
rebuild in the qtc::signature package.

Also the signature is made directly from signed_content_xml()
because having a checksum of a checksum will not make anything
better.