Print versions of models in MIX log

This PR is correspond to #479 

With this work, MIX log will be look like

```
I0205 21:42:20.555217 24446 linear_mixer.cpp:393] success to get_diff from [x.x.x.x:9198, y.y.y.y:9199]
I0205 21:42:20.557976 24447 linear_mixer.cpp:535] put_diff with 12 bytes finished my model is still up to date
I0205 21:42:20.559496 24446 linear_mixer.cpp:442] success to put_diff to [x.x.x.x:9198, y.y.y.y:9199]
I0205 21:42:20.559533 24446 linear_mixer.cpp:454] mixed with 2 servers in 0.009165 secs, 12 bytes (serialized data) has been put.
I0205 21:42:20.559563 24446 linear_mixer.cpp:324] .... mix done. versions[1, 0]
I0205 21:42:21.561661 24446 linear_mixer.cpp:305] starting mix:
I0205 21:42:21.566166 24446 linear_mixer.cpp:393] success to get_diff from [x.x.x.x:9198, y.y.y.y:9199]
I0205 21:42:21.568969 24448 linear_mixer.cpp:535] put_diff with 12 bytes finished my model is still up to date
I0205 21:42:21.570065 24446 linear_mixer.cpp:442] success to put_diff to [x.x.x.x:9198, y.y.y.y:9199]
I0205 21:42:21.570094 24446 linear_mixer.cpp:454] mixed with 2 servers in 0.00839 secs, 12 bytes (serialized data) has been put.
I0205 21:42:21.570124 24446 linear_mixer.cpp:324] .... mix done. versions[2, 0]
```

the version number displayed and copied among jubatus cluster.