tools: add etcd-dump-logs

The tool can dump the log from data directory.
It helps develop and debug.

fixes #1438 

output sample:
```
Snapshot: non-exist
WAL: nodeID=a8266ecf031671f3 clusterID=e9c7614f68f35fb2 term=2 commitIndex=10 vote=924e2e83e93f2560
WAL entries:
term	index	type	data
1	1	conf	type=ConfChangeAddNode id=6e3bd23ae5f1eae0
1	2	conf	type=ConfChangeAddNode id=924e2e83e93f2560
1	3	conf	type=ConfChangeAddNode id=a8266ecf031671f3
2	4	norm	noop
2	5	norm	method=PUT path="/0/members/924e2e83e93f2560/attributes" val="{\"name\":\"infra3\",\"clientURLs\":[\"http://localhost:4003\"]}"
2	6	norm	method=SYNC time="2014-12-30 14:44:40.484936961 -0800 PST"
2	7	norm	method=PUT path="/0/members/6e3bd23ae5f1eae0/attributes" val="{\"name\":\"infra2\",\"clientURLs\":[\"http://localhost:4002\"]}"
2	8	norm	method=PUT path="/0/members/a8266ecf031671f3/attributes" val="{\"name\":\"infra1\",\"clientURLs\":[\"http://localhost:4001\"]}"
2	9	norm	method=SYNC time="2014-12-30 14:44:40.78015987 -0800 PST"
2	10	norm	method=SYNC time="2014-12-30 14:44:41.280114316 -0800 PST"
```

/cc @kelseyhightower @robszumski 
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