allow verify/2 to accept atom for encryption type

ping @opscode/server-team 

In order to work around an issue in erlang compiler which prevents us
from specificying {d, value, <<"abinary">>}.  as a compiler option in rebar.config,
we needed to set default encryption types to be an atom. - this change
allows verify/2 to convert a received binary string to atom to prevent
match failures.
Do we originate HashType?  If not, are we leaking atoms based on possible user input?