Print the Jedi version when REPL completion is used

```
[repl_info]danilo@x2000:jedi$ PYTHONSTARTUP="/home/danilo/Projects/jedi/jedi/api/replstartup.py" python
Python 3.4.0 (default, Mar 17 2014, 23:20:09) 
[GCC 4.8.2 20140206 (prerelease)] on linux
Type "help", "copyright", "credits" or "license" for more information.
REPL completion using Jedi 0.8.0-final0
>>> 
```

This also makes debugging easier, because people see which completion they're actually using.

Maybe there should be a way to turn it off though? Is this even useful at all?