Add extras_require to setup.py

``` python
extras_require = {
    # TODO revisit
    'backends': ['odo', 'pywebhdfs', 'pymongo', 'sqlalchemy', 'paramiko',
                 'cassandra-driver'],
    # dask.mesos should be enough
    'mesos': ['dask.mesos', 'satyr', 'toolz', 'mesos.native'],
    'spark': ['pyspark', 'py4j']
}
extras_require['complete'] = sorted(set(sum(extras_require.values(), [])))
```
