*: init interface
Merge pull request #1 from go-distributed/interface

*: init interface
*: add framework interface
Merge pull request #2 from go-distributed/framework

*: add framework interface
*: add a node interface
Merge pull request #3 from go-distributed/node

*: add a node interface
refactor the topology
add comments for topology usage
add comments for topology usage
add comments for topology usage
Merge pull request #4 from xiaoyunwu/master

refactor the topology. Thanks Xiaoyun.
etcd layout: initial definition