Add the ASN.1 structures and functions for CertificatePair, which is

defined as follows (according to X.509_4thEditionDraftV6.pdf):

CertificatePair ::= SEQUENCE {
	forward		[0]	Certificate OPTIONAL,
	reverse		[1]	Certificate OPTIONAL,
	-- at least one of the pair shall be present -- }

The only thing I'm not sure about is if it's implicit or explicit tags
that I should count on.  For now, I'm thinking explicit, but will
gladly stand corrected.

Also implement the PEM functions to read and write certificate pairs,
and defined the PEM tag as "CERTIFICATE PAIR".

This needed to be defined, mostly for the sake of the LDAP attribute
crossCertificatePair, but may prove useful elsewhere as well.