fix(*.go): rename HeartbeatTimeout to HeartbeatInterval

While re-reading the etcd tuning guide I realized that perhaps it is a
bit of a misnomer to call this a "Timeout" since unlike the "Election
Timeout" there is no immediate consequence of it being missed.

Perhaps naming it interval would be better?
Merge pull request #166 from philips/change-heartbeatTimeout-to-heartbeatInterval

fix(*.go): rename HeartbeatTimeout to HeartbeatInterval