Rename PIN_GOOGLE_CERT to PIN_GOOGLE_CA_CERT.

I think this makes it more clear that this certificate belongs to a CA
(i.e., Equifax), not to Google.
Remove unused M2Crypto.X509 imports.

It looks like all uses of X509 were removed in 454b735a.
Typo.
seek to the beginning after writing the temp file, since that is probably what users expect
add more documentation on inner vs outer transports