Default to Python 3

In support of #8.

Right now I've just installed it and I'm trying to understand the right way to default to python3. There is the option of symlinking both `python` and `pip`.
Actually @takluyver, get-pip automatically created `pip`.

```
root@05bf21b72ac8:/# pip -V
pip 1.5.6 from /usr/local/lib/python3.4/dist-packages (python 3.4)
```