Make facilitator public key configurable

The FACILITATOR_PUBLIC_KEY is hardcoded in the registration helpers flashproxy-reg-email and flashproxy-reg-url. It won't work for any facilitator but the one at fp-facilitator.org.

It should be configurable from the command line so that you can easily test with another facilitator.

./flashproxy-reg-email --facilitator-pubkey key.asc

This configurable public key corresponds to the private key that is given to flashproxy-reg-daemon with the --key option.