Bridge certificate: set end date to start date if PGP key doesn't expire

This will of course need to be checked server-side because it's not a
standard way of using expiration information.

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Export keys in PEM/armored and as PKCS#12 pack (closes #93)

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>