add capability to bootstrap from DNS
etcdctl: add environment support to certs args
docs: fix port in peer URLs
Merge pull request #1955 from coreos/doc-cleanup

docs: fix port in peer URLs
doc: update node migration guide
Merge pull request #1956 from coreos/member-migration-example

doc: update node migration guide
doc: add backword_compatibility.md
Merge pull request #1951 from jlsalvador/patch-1

etcdctl: add environment support to certs args
Merge pull request #1958 from xiang90/compatibility

doc: add backword_compatibility.md
Add a simple test and mock for genDNS
fix from comments
add apurl checking and logging
Documentation: update to 2.0

In anticipation for a 2.0.0-rc.0 release update and move the
documentation.
version: bump to v2.0.0-rc.1
logos: add SVG and PNG logos
Merge pull request #1963 from robszumski/add-logo

logos: add SVG and PNG logos
raft: leader waits for the reply of previous message when follower is not in good path.

It is reasonable for the leader to wait for the reply before sending out the next
msgApp or msgSnap for the follower in bad path. Or the leader will send out useless
messages if the previous message is rejected or the previous message is a snapshot.
Especially for the snapshot case, the leader will be 100% to send out duplicate message
including the snapshot, which is a huge waste.

This commit implement a timeout based wait mechanism. The timeout for normal msgApp is a
heartbeatTimeout and the timeout for snapshot is electionTimeout(snapshot is larger). We
can implement a piggyback mechanism(application notifies the msg lost) in the future
if necessary.
readme: add logo
Merge pull request #1962 from xiang90/raft

raft: leader waits for the reply of previous message
Fully resolve DNS entries to IPs and ignore single errors (such as no etcd-server-ssl)