Fix data installation.

The setup should use package_data instead of data_files.

data_files are supposed to be outside the package. Plus the installation didn't work as seen hereunder:
```
installing package data to build/bdist.macosx-10.9-x86_64/egg
running install_data
warning: install_data: setup script did not provide a directory for 'rest_framework_bulk' -- installing right in 'build/bdist.macosx-10.9-x86_64/egg'

error: can't copy 'rest_framework_bulk': doesn't exist or not a regular file
```

package_data should be used instead to place binary.
Also I changed a bit the get_package_data so it skips the __pycache__ directory.