add isMonitor attribute to DisplayPolicy model
change serval methods in DisplayPolicyManager
refactor HeartbeatAlert
refactor Heartbeat history part
Merge branch 'master' of github.com:dianping/cat

Conflicts:

cat-home/src/main/java/com/dianping/cat/report/page/heartbeat/HistoryGra
phs.java
Merge pull request #682 from leonindy/master

refactor heartbeat history part