Allow usage of etcd serializable reads

[tanguy-ortolo]

Hello,

I noticed that easyKV uses linearizable reads with both etcd v2 (where is is specified as an option Quorum: true) and with etcd v3 (where it is the implicit default).

Linearizable reads are interesting, but come with a latency cost, which not everybody is willing to pay. I think it would be good to allow this to be configured, I am therefore submitting PR #14.

[tanguy-ortolo]

A corresponding request for remco will follow shortly. :-)

Be aware that, for remco to use that new feature, you will have to tag a new version of easyKV first.