arclib directory is supposed to include classes which will be used as convinient 
lib for client and service.
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, 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 voms api can still be used if need to process the voms extension).
The Crendential class support PEM, DER PKCS12 credential.

