Alternative fix to #630 - allow uses to specify whether to use existing distribute installation

The issue #630 is trying to fix is that if distribute is already installed, but is older than the requirement in ``distribute_setup.py`` we don't necessarily want to download it (e.g. for package managers). The approach there was to try importing setuptools in a process to check if it was distribute or vanilla setuptools, and then acting accordingly - hacky to say the least.

Here is a much cleaner option - we allow users to force the installer to use an existing system installation of distribute with the ``--use-system-distribute`` option, i.e.:

```
python setup.py build --use-system-distribute
```

this is in line with what we do for other external libraries/modules.

@iguananaut @mdboom @eteq - any thoughts on this?