Change core.Certificate.DER to []byte.
Change core.Certificate.DER to []byte.

Fixes https://github.com/letsencrypt/boulder/issues/519.

The previous type, JSONBuffer, was triggering a subtle bug when scanning
multiple rows from MySQL.  Since this struct is not serialized as JOSE it
doesn't need to have the JSONBuffer type.

The test for this fix is blocked on
https://github.com/letsencrypt/boulder/issues/132, so I filed a separate issue
to follow up with a test:
https://github.com/letsencrypt/boulder/issues/536

jfrazelle/boulder,hlandauf/boulder,kuba/boulder,jcjones/boulder,jsha/boulder,hlandauf/boulder,jgillula/boulder,mommel/boulder,ibukanov/boulder,benileo/boulder,josephyzhou/boulder,tomclegg/boulder,julienschmidt/boulder,tomclegg/boulder,devpaul/boulder,jmhodges/boulder,KyleChamberlin/boulder,jmhodges/boulder,lucasallan/boulder,lmcro/boulder,jfrazelle/boulder,ibukanov/boulder,lucasallan/boulder,modulexcite/boulder,tomclegg/boulder,lmcro/boulder,ZCloud-Firstserver/boulder,KyleChamberlin/boulder,devpaul/boulder,devpaul/boulder,sophie-h/boulder,ZCloud-Firstserver/boulder,mozmark/boulder,julienschmidt/boulder,ZCloud-Firstserver/boulder,lucasallan/boulder,mommel/boulder,julienschmidt/boulder,lmcro/boulder,julienschmidt/boulder,jcjones/boulder,mehmooda/boulder,jgillula/boulder,mozmark/boulder,mozmark/boulder,mommel/boulder,sophie-h/boulder,kuba/boulder,ibukanov/boulder,jsha/boulder,bmw/boulder,sophie-h/boulder,r0ro/boulder,jmhodges/boulder,deserted/boulder,jsha/boulder,mommel/boulder,r0ro/boulder,mehmooda/boulder,josephyzhou/boulder,mehmooda/boulder,tomclegg/boulder,ZCloud-Firstserver/boulder,kuba/boulder,mozmark/boulder,deserted/boulder,patf/boulder,KyleChamberlin/boulder,r0ro/boulder,benileo/boulder,postfix/boulder,devpaul/boulder,jcjones/boulder,jgillula/boulder,jmhodges/boulder,postfix/boulder,lmcro/boulder,postfix/boulder,hlandauf/boulder,modulexcite/boulder,patf/boulder,letsencrypt/boulder,deserted/boulder,josephyzhou/boulder,modulexcite/boulder,hlandauf/boulder,josephyzhou/boulder,r0ro/boulder,jfrazelle/boulder,bmw/boulder,kuba/boulder,postfix/boulder,KyleChamberlin/boulder,deserted/boulder,benileo/boulder,bmw/boulder,benileo/boulder,patf/boulder,jcjones/boulder,mehmooda/boulder,jgillula/boulder,modulexcite/boulder,letsencrypt/boulder,letsencrypt/boulder,patf/boulder,jfrazelle/boulder