Issue6

Added ability to to serval-sign to choose between keyring files (to keep identity lists separate). If no keyring file is specified, serval-sign will use the default "serval.keyring" file. If specified keyring does not exist, it will be created.

To test: (remember to be root and have servald running)
1) ./serval-sign --help 
You should see the -k flag along with an explanation of its functionality
2) ./serval-sign -k test message
You should see the message "message" followed by the signature block
3) sudo ls /var/serval-node/
You should see "test" in this directory. It is the new keyring you created

Keep out of reach of children.