Merge pull request #4 from xiaoyunwu/master

refactor the topology. Thanks Xiaoyun.
etcd layout: initial definition
Merge pull request #5 from fengjingchao/master

etcd layout: initial definition
add numberOfTasks interface to topology
Merge pull request #6 from go-distributed/f

add numberOfTasks interface to topology
add tree_topo.go
Merge pull request #8 from go-distributed/topo_impl

add tree_topo.go
Merge remote-tracking branch 'upstream/master'
add test to tree_topo