Display python version & platform at repl startup

Displays the python version & platform information at repl startup something like
```
hy 0.10.0 using Python 3.3.3 (default, Dec  8 2013, 14:51:59) 
[GCC 4.8.2] on linux
=>
```
Mostly trivial info, and probably only makes sense if you end up creating virtualenvs\ pip installing and don't remember which python ;)