Add which() function to pake.py

This PR adds a ```which()``` function to ```pake.py```

For more information, see https://github.com/openlayers/ol3/pull/448

To create this PR, I just copy-paste ```pake.py``` from the upper OL3 PR.

Surprisingly I had other lines which did not match between the
```ol3/pake.py``` version and the one in this repo. I inserted a snapshot hereunder.

The current PR only implements the ```which()``` function, but *not* the
other differences!

@twpayne, what should I do with these differences ? If not added to ```pake.py```,
you're going to have a problem next time you update ```ol3/pake.py```

![snap](https://f.cloud.github.com/assets/1681332/329643/a5f9dc78-9bb8-11e2-978d-971d1eff6b14.png)