tools: add etcd-dump-logs

The tool can dump the log from data directory.
It helps develop and debug.

fixes #1438 

output sample:
```
Snapshot:
empty
WAL metadata:
nodeID=a8266ecf031671f3 clusterID=e9c7614f68f35fb2 term=2 commitIndex=10 vote=924e2e83e93f2560
WAL entries:
lastIndex=10
term	     index	type	data
   1	         1	conf	method=ConfChangeAddNode id=6e3bd23ae5f1eae0
   1	         2	conf	method=ConfChangeAddNode id=924e2e83e93f2560
   1	         3	conf	method=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"
```

and

```
Snapshot:
term=3 index=11915 nodes=[6e3bd23ae5f1eae0 924e2e83e93f2560 a8266ecf031671f3]
WAL metadata:
nodeID=a8266ecf031671f3 clusterID=e9c7614f68f35fb2 term=3 commitIndex=16185 vote=924e2e83e93f2560
WAL entries:
lastIndex=16185
term	     index	type	data
   3	     11916	norm	method=PUT path="/1/bench" val=""
   3	     11917	norm	method=PUT path="/1/bench" val=""
   3	     11918	norm	method=PUT path="/1/bench" val=""
   3	     11919	norm	method=PUT path="/1/bench" val=""
   3	     11920	norm	method=PUT path="/1/bench" val=""
   3	     11921	norm	method=PUT path="/1/bench" val=""
   3	     11922	norm	method=PUT path="/1/bench" val=""
   3	     11923	norm	method=PUT path="/1/bench" val=""
   3	     11924	norm	method=PUT path="/1/bench" val=""
   3	     11925	norm	method=PUT path="/1/bench" val=""
   3	     11926	norm	method=PUT path="/1/bench" val=""
   3	     11927	norm	method=PUT path="/1/bench" val=""
   3	     11928	norm	method=PUT path="/1/bench" val=""
   3	     11929	norm	method=PUT path="/1/bench" val=""
   3	     11930	norm	method=PUT path="/1/bench" val=""
   3	     11931	norm	method=PUT path="/1/bench" val=""
   3	     11932	norm	method=PUT path="/1/bench" val=""
   3	     11933	norm	method=PUT path="/1/bench" val=""
   3	     11934	norm	method=PUT path="/1/bench" val=""
   3	     11935	norm	method=PUT path="/1/bench" val=""
   3	     11936	norm	method=PUT path="/1/bench" val=""
   3	     11937	norm	method=PUT path="/1/bench" val=""
....
```

/cc @kelseyhightower @robszumski