Upload support

This pull request introduces upload support. It does not come with documentation, as that will be part of a later pull request. If you wish to test uploading an archive: find any Python package, and run it's setup.py with the sdist argument. It will output a tarball, and you should be able to upload that to Pulp. To verify that it's been uploaded, you should be able to see it in Mongo like this:

```
> db.units_python_package.find()
```

Also, you should find the package you uploaded stored in ```/var/lib/pulp/content/python_package/```.
Same as above.