config: save local uid.

So peer can know that we are the same machine.
Support user defined ip addresses.

User can define peer ip in json file.
Use '-p ip_config.json' to load the ip configuration.
Format of ip_config.json (An json array):

[
        {
            "uid": "54f2b14f80e5374f8af7ad1b4838145ea7eaaf57",
            "ipv4": "172.31.0.101",
            "desc": "Linux VM"
        }
]
fixup bug when assign ip address.
Merge pull request #9 from kangjianbin/master

Save local uid and add an option to load peer uid<->ip mapping configuration