Improve clarity of ?keys behaviour.

I read the previous version initially as meaning that it would return a list of keys which have no value set, which I couldn't understand. I think this removes some ambiguity.
Seems like we should actually check the reference count.
Merge pull request #210 from robholland/patch-1

Improve clarity of ?keys behaviour.
Merge pull request #212 from rbg/master

Seems like we should actually check the reference count.
Cutting v0.3.0
Adding the build versions
Adding server_name configuration for TLS
agent: Copy ServerName config
Add expect bootstrap '-expect=n' mode.

This allows for us to automatically bootstrap a cluster of nodes after
'n' number of server nodes join. All servers must have the same 'n' set, or
they will fail to join the cluster; all servers will not join the peer set
until they hit 'n' server nodes.

If the raft commit index is not empty, '-expect=n' does nothing because it
thinks you've already bootstrapped.

Signed-off-by: Robert Xu <robxu9@gmail.com>