Minimal implementation of implicit references

Following the discussion in #30 I have implemented minimal implicit Refs, so we are able to write stuff like this:

```python
t = Template()
image = Parameter("ImageId", Type="String")
t.add_parameter(image)
r = ec2.Instance('MyInstance', ImageId=image)
t.add_resource(r)
```

instead of

```python
t = Template()
image = Parameter("ImageId", Type="String")
t.add_parameter(image)
r = ec2.Instance('MyInstance', ImageId=Ref(image))
t.add_resource(r)
```

We could make this feature optional via module-level var, like this(not yet implemented):

```
import troposphere
troposphere.use_implicit_ref = True
```