Make installable with pip

This will at least make the project installable with pip. Maybe it will facilitate getting it hosted on PyPI.

Once merged you could install directly from github with:
```shell
pip install git+git://github.com/jeffkaufman/icdiff.git@master
```

Since the changes are already hosted on github, you could try it out by installing from my branch like this:
```shell
pip install git+git://github.com/macro1/icdiff.git@package
```

If you've installed it, and want to try installing again, you can `pip uninstall icdiff` before rerunning the install command.