version: bump to v0.5.2+git
pkg: add ExpBackoff function
heart: add Heart & Monitor structs
server: use heart.Monitor to monitor etcd connectivity
API: wrap handlers with availability
Merge pull request #611 from bcwaldon/heart-monitor

Monitor etcd connectivity
agent: stop units in goroutine during init

Fix #624
Merge pull request #630 from bcwaldon/624

agent: stop units in goroutine during init
systemd: subscribe while unit should exist