credential directory is supposed to include classes which will be 
used as convinient lib for credential handling.


Credential class covers the functionality about general 
processing to certificate/key files, including cerficate/key 
parsing, information extracting (such as subject name, 
issuer name, lifetime, etc.), chain verifying, extension 
processing about proxy certinfo, extension processing about 
other general certificate extension (such as voms attributes
certificate).

It should be the extension-specific code itself (for voms, it 
is some code about writing and parsing voms-implementing 
Attibute Certificate/RFC3281, the voms-attibute is then 
be looked as a binary part and embeded into extension
of X509 certificate/proxy certificate) to create, parse and 
verify the extension, not the Credential class.

The VOMSUtil code is used to parse the voms AC extension.

The Crendential class support PEM, DER PKCS12 credential.

