fix(README): use machine instead of node

Internally we call every member of an etcd cluster a "machine" so stay
consistent when talking in the README.
refactor(Node) do not expose node struct
refactor event
refactor change node_repr to node_extern
Merge pull request #342 from philips/use-machine-nomenclature

fix(README): use machine instead of node
fix(README): fix 9 instances of v1

remove references to the v1 API and consistently use the v2 API.
Merge pull request #330 from philips/fixup-readme-remove-v1

fix(README): fix 9 instances of v1
fix(event_history.go) should not scan prefix
Fix spelling mistakes in internal-protocol-versioning.md
fix(README): repair some noisy spaces
Merge pull request #353 from baruch/patch-1

Fix spelling mistakes in internal-protocol-versioning.md
feat(README): add a prevExist example

On the mailing list Dustin Oprea suggested adding a prevExist concrete
example would make it more clear that the value must be true or false.
fix tests bump deps
test add watcher prefix test
Merge pull request #352 from xiangli-cmu/fix_watch_prefix

fix(event_history.go) should not scan prefix
fix root should be rdonly
tests add root readonly test
Merge pull request #354 from philips/fixup-prevExist-README

feat(README): add a prevExist example
Merge pull request #355 from xiangli-cmu/root_readonly

WIP fix root should be rdonly
fix(server/v2): redirect to ClientURL not PeerURL

If consistent is set you must redirect the client to the leader's
ClientURL not the PeerURL.