Add all dependencies to install_requires.

Fixes #558.
Good idea, but feature names are a bit long to write in my opinion. For example:

```sh
$ pip install luigi[luigi.server]
```

I'd prefer shorters names:

```py
    extras_require = {
        'es': ['elasticsearch>=1.0.0'],
        'mysql': ['MySQL-python'],
        'redshift': ['pyscopg2'],
        'task_history': ['sqlalchemy'],
        'postgres': ['psycopg2'],
        's3': ['boto'],
        'server': ['tornado'],
        'webhdfs': ['whoops'],
    }
```

What do you think?