Merge pull request #11 from yifan-gu/dev

after discussion, agree on merge
finish union and very simple recvPreAcceptReply
Merge branch 'dev' of github.com:go-epaxos/epaxos into dev

Conflicts:
	replica/instance.go
	replica/phase1_test.go
fix update and allReplyTheSame
refactor ballot
Merge pull request #13 from fengjingchao/dev

before finishing preaccept reply
Merge branch 'dev' of github.com:go-epaxos/epaxos into dev
add updating MaxInstanceNum when receiving accept and commit