Documentation: Clarify etcd security documentation

Signed-off-by: Vincent Ambo <vincent@kivra.com>
raft: prev should be set only when we sucessfully send out rd to the channel
raft: record previous state is enough
raft: set ready.st to empty state, if current state is equal to previous state
wal: do not save empty state
raft: add a newReady helper function
raft: remove extra code
wal: test save empty state
scripts: use bash for build script
raft: use IsEmptyState
Merge pull request #1026 from coreos/fix_node

Fix node
raft: do not need to copy committed entries
Merge pull request #1032 from jonboulle/fix_tests

scripts: use bash for build script
gitignore: add _etcd_data directory ignores

After 9a57d1067d8613cd5aa8bd6e00b7522553784770 we want to ignore all of
these _etcd_data directories.
Merge pull request #1036 from philips/gitignore-etcd-data

gitignore: add _etcd_data directory ignores
etcdhttp: add /v2/machines endpoint
http: allow GET, HEAD for /v2/machines
build: add travis configuration
http: add allow func

For further extendability
Merge pull request #1037 from jonboulle/travis

build: add travis configuration