Adding setup.py so it is installable via pip, etc


`pip install git+http://github.com/GoogleCloudPlatform/endpoints-proto-datastore.git#egg=endpoints-proto-datastore`
This is a nitpick (not a big deal) but as far as namespaces, the preference is
```python
import setuptools
```
and then here use
```python
setuptools.setup(
    ...
```
and later use
```python
setuptools.find_packages(exclude=['examples*', 'docs*'])
```

I believe I formed this preference from within Google, not actually from PEP8.