fix(discovery): use prevExist instead of prevValue=init

Use PUT /_state?prevExist=true in the protocol instead of PUT
/_state?prevValue=init. This lets people point one vanilla etcd at the
key prefix of another vanilla etcd and have it just work.
Merge pull request #534 from philips/discovery-protocol-fix

fix(discovery): use prevExist instead of prevValue=init
feat(config): remove the info file

The info file was meant to help the user from accidently making a
mistake but often times it just confuses people:

https://github.com/coreos/etcd/issues/356
https://github.com/coreos/etcd/issues/531
https://github.com/coreos/etcd/issues/318

Lets remove the info file for this next release.
Merge pull request #532 from philips/remove-info-file

feat(config): remove the info file
Merge pull request #533 from robszumski/master

fix(docs): add full command example and link to discovery spec
fix(docs): explicitly create links instead of using the markdown parser