Allow instance overrides for cert and password

I needed to be able to create passes for multiple certs, which meant that having a module level attribute for the cert and password would not work.  What I've done, is allowed the cert and password to be set on the PKPass itself.  If they exist, they're used.  Otherwise it falls back to the existing functionality.

I did not do this for the scenario where the p12_key file is owned, because it didn't seem like a likely scenario.  Easy enough to add though if anyone feels there'd be some value.

-- Sorry about the branch name...  not sure what happened there.