Can't use variable names in the create block

The following code produces an error.

```
---
input_name: some/stupid/filename.txt

tasks:

  -
    creates: {{input_name}}
    command:
      - mkdir -p $(dirname {{creates}})
      - wget http://en.wikipedia.org/wiki/List_of_S%26P_500_companies -O {{creates}}
    alias: get_data
```

Traceback (most recent call last):
  File "/usr/local/bin/workflow", line 7, in <module>
    execfile(__file__)
  File "/Users/aaron/src/workflow/bin/workflow", line 30, in <module>
    available_tasks = get_available_tasks()
  File "/Users/aaron/src/workflow/workflow/parser.py", line 86, in get_available_tasks
    task_graph = load_task_graph()
  File "/Users/aaron/src/workflow/workflow/parser.py", line 56, in load_task_graph
    for i, yaml_obj in enumerate(config_yaml):
  File "/Library/Python/2.7/site-packages/yaml/__init__.py", line 83, in load_all
    yield loader.get_data()
  File "/Library/Python/2.7/site-packages/yaml/constructor.py", line 33, in get_data
    return self.construct_document(self.get_node())
  File "/Library/Python/2.7/site-packages/yaml/constructor.py", line 48, in construct_document
    for dummy in generator:
  File "/Library/Python/2.7/site-packages/yaml/constructor.py", line 398, in construct_yaml_map
    value = self.construct_mapping(node)
  File "/Library/Python/2.7/site-packages/yaml/constructor.py", line 208, in construct_mapping
    return BaseConstructor.construct_mapping(self, node, deep=deep)
  File "/Library/Python/2.7/site-packages/yaml/constructor.py", line 132, in construct_mapping
    "found unacceptable key (%s)" % exc, key_node.start_mark)
yaml.constructor.ConstructorError: while constructing a mapping
  in "<string>", line 17, column 14:
        creates: {{input_name}}
                 ^
found unacceptable key (unhashable type: 'dict')
  in "<string>", line 17, column 15:
        creates: {{input_name}}