Add mock to __init__.py so it can be used when using "import luigi".

Hello! 

I think it would be great if the mock module could be from the luigi namespace. For example

```python
import luigi
hoho_mock = luigi.mock.MockFile('hoho')
```

Now I do the following because I'm using more than just the mock module.

```python
import luigi
from luigi import mock
hoho_mock = mock.MockFile('hoho')
# further down
luigi.build[something], local_scheduler=True, test=True)
```

Another area where it's also good to keep it under the luigi namespace is when using the popular [Mock framework](http://www.voidspace.org.uk/python/mock/). I would prefer to not do for example this workaround.
 
```python
import luigi
from luigi import mock as luigi_mock
import mock
hoho_mock = luigi_mock.MockFile('hoho')
# further down
mock = mock.Mock()
```


P.S. Thanks for Luigi. It's great. :+1: