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superseded by -advertise-client-urls, -listen-client-urls,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
pkg/types: introduce a URLs type

Cleanup the usage of URLs into its own type so we don't have to use a
FlagValue everywhere we have a list of URLs.
Merge pull request #1221 from philips/fixing-stuff

pkg/types: introduce a URLs type
raft: add removed

The usage of removed:
1. tell removed node about its removal explicitly using msgDenied
2. prevent removed node disrupt cluster progress by launching leader election

It is set when apply node removal, or receive msgDenied.
raft: refine comment for doc and removed list tests
etcdserver: fix test due to rebase