Add JsonErrorMixin

This commit adds the JsonErrorMixin for formatting error message from a
given RequestHandler as JSON.
bah ... thwarted by tornado again.  What about a structure like:
````
HTTP/1.1 400 Something's happening here.
Content-Type: application/json; charset=utf-8

{
    "type": "ExceptionClass",
    "documentation_url": "http://www.aweber.com/optional/documentation",
    "message": "Error message from the exc_value.get_message()"
}
````

`type` would be the first element in the `exc_info` tuple or maybe even `null` if the information is not available.

`message` would be the result of calling `raised_error.get_message()` or `'Unexpected error'`.