Add all dependencies to install_requires.

Fixes #558.
several of these are optional requirements. We could specify them that way in case people don't want them, e.g.:

```
    extras_require = {
        'luigi.contrib.esindex': ['elasticsearch>=1.0.0'],
        'luigi.contrib.mysqldb': ['MySQL-python'],
        'luigi.contrib.redshift': ['pyscopg2'],
        'luigi.db_task_history': ['sqlalchemy'],
        'luigi.postgres': ['psycopg2'],
        'luigi.s3': ['boto'],
        'luigi.server': ['tornado'],
        'luigi.contrib.webhdfs': ['whoops'],
    }
```

See https://pythonhosted.org/setuptools/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies