rename monitor and delta attributes in DisplayPolicy
add extension metrics when setting heartbeat alert rule
rename monitor attribute of DisplayPolicy
refactor heartbeat alert
refactor alert module: extract RuleConfigManager to a abstract method
Merge branch 'master' of github.com:dianping/cat
Merge pull request #683 from leonindy/master

refactor heartbeat alert, make a change of RuleConfigManager in alert module
merge code