Support for scheduled jobs via fabric_rundeck.

Should also fix #1. Ping @balanced-cookbooks/chefs.

An example from the test fabfile:

```python
from fabric.api import task, local
from fabric.decorators import roles

from fabric_rundeck import cron

@cron('0 12 * * *')
@roles('www')
@task
def one():
    """Task one."""
    local('echo one')
```