added prevExist flag to CAS

[jeremyplichta]

Added the ability to specify prevExist to CAS

[jeremyplichta]

The test fails because the latest etcd cloned from github now returns this from /version instead of a plain string of etcd 0.*

{"releaseVersion":"2.0.0-rc.1","internalVersion":"2"}

I can update the test to expect that instead, but not sure if there are other consequences this version change might have on the library.