Support for multiple keys / seamless key rotation

[jedisct1]

Accept multiple keys.

I still need to update the README file, but here is how to seamlessly switch to a new key:

1) Create the first key and its related certificate:

$ dnscrypt-wrapper --gen-crypt-keypair --crypt-secretkey-file=1.key
$ dnscrypt-wrapper --gen-cert-file --crypt-secretkey-file=1.key --provider-cert-file=1.cert

Run the server:

$ dnscrypt-wrapper --resolver-address=114.114.114.114 \
                   --provider-name=2.dnscrypt-example.org \
                   --listen-address=0.0.0.0:443 \
                   --crypt-secretkey-file=1.key \
                   --provider-cert-file=1.cert

2) Before 1.key expires, create a fresh new key and a certificate for it:

$ dnscrypt-wrapper --gen-crypt-keypair --crypt-secretkey-file=2.key
$ dnscrypt-wrapper --gen-cert-file --crypt-secretkey-file=2.key --provider-cert-file=2.cert

Run a new instance of the server, which is going to publish the certificate
for the new key, but still accept queries using the previous key in addition
to the new one (notice the --crypt-secretkey-file= line, which can now include
an arbitrary number of keys):

$ dnscrypt-wrapper --resolver-address=114.114.114.114 \
                   --provider-name=2.dnscrypt-example.org \
                   --listen-address=0.0.0.0:443 \
                   --crypt-secretkey-file=1.key,2.key \
                   --provider-cert-file=2.cert

3) Wait 1 hour and remove the old key:

$ dnscrypt-wrapper --resolver-address=114.114.114.114 \
                   --provider-name=2.dnscrypt-example.org \
                   --listen-address=0.0.0.0:443 \
                   --crypt-secretkey-file=2.key \
                   --provider-cert-file=2.cert

[pysiak]

This is fantastic! Thanks!