main: retrieve config from env vars
etcdserver: use Action to fix possible data race
*: init for on disk snap support
Merge pull request #1095 from coreos/nsnap

WIP *: init for on disk snap support
main: add test for setting flags
raft: introduce Node interface
Merge pull request #1093 from jonboulle/node_interface

Create Node Interface
Merge pull request #1097 from unihorn/130

etcdserver: use Action to fix possible data race
wal: OpenFromIndex fails if it cannot find previous index

Example:
We save entry 1, 2, 3 to WAL.
If we try to open 100, it should fail.
Merge pull request #1040 from jonboulle/1040_config_env_vars

etcd needs to be configurable through env vars
etcdserver: use simple PRNG for GenID
wal: add ErrFileNotFound and ErrIndexNotFound
Merge pull request #1092 from jonboulle/dumb_gen_id

GenID: use a fast prnd
Merge pull request #1099 from unihorn/128

wal: OpenFromIndex fails if it cannot find previous index
raft: add exclusive entry

Configuration change entry needs to be exclusive entry for safety.