flags: replace Addrs with IPAddressPort
pkg/flags: add NewURLs helper
pkg: add URLsFromFlags
etcd: support v0.4.6 addr flags

The -addr, -bind-addr, -peer-addr, and peer-bind-addr flags are
superseded by -advertise-client-urls, -listen-client-urls,
-advertise-peer-urls, and -listen-peer-urls, respectively.
If any of the former flags are provided, however, they will still
work. If the new counterparts to the old flags are provided, etcd
will log an error and fail.
Merge pull request #1220 from bcwaldon/bkcompat

Backwards-compatibility with v0.4.6 addr-related flags