allow verify/2 to accept atom for encryption type

In order to work around an issue in erlang compiler which prevents you
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.
Merge pull request #110 from opscode/mp/authenticate_endpoint_feature_parity

allow verify/2 to accept atom for encryption type