Add CRL checking for client certificate

If the client provides a certificate, make sure it is actually valid AND
not revoked.
Reformatting.

Add comments; verify clean Dialyzer, test against Riak Test, and
document SSL functions.
Merge pull request #40 from basho/bugfix/csm/reformatting-adt-pb-crl

Reformatting.
Merge pull request #39 from basho/adt-pb-crl

Add CRL checking for client certificate