add watcher for expiration, add expiration for TestAndSet, add related test case
add ExpireCount and some test case
add LastIndex and LastTerm in EventHistory
fix race between Expire() and others, fix UpdateTTL(), modified watcher to catch Expire()
delete some debug comments in stats_test.go
rename to DupCnt, duped; add some comments, maintained some format, add notification for immediate expiration
add some comment and change a declaration form
Merge pull request #15 from evan-gu/newStore

add watcher for expiration, add expiration for TestAndSet