display plugin version on boot

fix #740 

Now, you only have to implement `version()` function.
Jubatus will display its version on startup message.

Version itself is only a `std::string`.
So users can use commit-hash or arbitrary string as a version.

### before

```
INFO  [dynamic_loader.cpp:87] plugin loaded: /usr/local/lib/jubatus/plugin/libmecab_splitter.so.0.6.1
```

### after

```
INFO  [dynamic_loader.cpp:87] plugin loaded: /usr/local/lib/jubatus/plugin/libmecab_splitter.so.0.6.1 version: 0.6.1
```