check type of task explicitly

Hi,

I'd like to propose a patch for this kind of stack one receives in case of wrong item type in playbook tasks list such as:
```yaml
tasks:
- include path.yml
```
Results to:
```
Traceback (most recent call last):
  File "/usr/bin/ansible-playbook", line 209, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/usr/bin/ansible-playbook", line 180, in main
    pb.run()
  File "/usr/lib/python2.6/site-packages/ansible/playbook/__init__.py", line 202, in run
    play = Play(self, play_ds, play_basedir)
  File "/usr/lib/python2.6/site-packages/ansible/playbook/play.py", line 83, in __init__
    self._tasks      = self._load_tasks(self._ds.get('tasks', []))
  File "/usr/lib/python2.6/site-packages/ansible/playbook/play.py", line 110, in _load_tasks
    tokens = shlex.split(x['include'])
TypeError: string indices must be integers, not str
```
Patched:
```
ERROR: expecting dict; got: include path.yml
```

Thanks!