Clean up (roll back) on keybard interrupt.

Hi @mitsuhiko,

I found that pressing `Ctrl` - `C` while `pipsi install foo` executing will dirty the package path. In next trying, pipsi will show "foo is already installed" because the package directory is exists (and empty).

I patched the `pipsi.py` by catching the `KeyboardInterrupt` and cleaning up.

Could you review it? Thank you.