Add config file support for distributed GMW

The config file tells what IP address/port each party can be contacted
on.

For example, the following config file

    foo:2000
    bar:3000
    baz:4000

specifies that there are three parties, party 0 at foo:2000, party 1 at
bar:3000, and party 3 at baz:4000, where foo, bar, and baz are
hostnames.  You can also use IP addresses (127.0.0.1) instead of
hostnames.  If there are N parties then each host uses N ports, e.g.,
foo:2000, foo:2001, foo:2002 for the above config file.

If that file is named config.gmw then party 0 can be started with

    go run vickrey.go -config config.gmw -id 0 8

for the vickrey example.  Here 8 is the input of party 0.  Of course
party 0 should run on host foo, party 1 should be started on host bar,
etc.