Feature/json resource

Make it possible to fo a GET that returns handler info as JSON. It is conceptually similar to the debug request in that it does not make a subsequent upstream call. The motivation is that an application can make a call to the deviceproxy service outside the request-response loop.
I'd suggest we make it mandatory for the handlers to return unicode strings. If we encounter something non-unicode then we need to raise an error.

This implementation is likely going to allow for very subtle bugs.

```python
>>> unicode(None)
u'None'
>>> None.encode('utf-8')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'encode'
>>>
```