Add a function to compute a sha256 hash out of a buffer
Allow to check rsa from buffer
New fingerprint kind of signature

if a repository is declared using the fingerprint type of signature.
a fingerprint directory should also be provided. Inside that fingerprint
directory should be found 2 directories trusted and revoked.
Those directories will contains files (in yaml format) containing the
fingerprint of the trusted certificated and the revoked ones:

--- begins --
function: sha256
fingerprint: an_hex_representation_of_the_fingerprint
--- ends ---

All unknown attribute will be silently ignored to allow extending in the
futur

It also support multiple signatures inside the catalog. the catalog
should provide the signature in the following form:
name.sig
name.cert

Discussed at:	BSDCam
Discussed with:	des, jonathan, gavin