Drop `ensure_ascii` from json.dumps() calls.

With `ensure_ascii` (the default), we observed the following in our
graphite installation when calling the /metrics/index.json resource:

        UnicodeDecodeError: 'utf8' codec can't decode byte 0x8a in position 81: invalid start byte
Add json_response_for().

Builds an HttpResponse instance with appropriately serialized JSON depending on the request's Accept header.
Use json_response_for() in the easy cases.
Return a dict in tree_json(), pass it to json_response_for().
Use json_response_for() for jsonp generation, too.
Kill dumps(), replaced with json_response_for().
Merge pull request #286 from whilp/dont-ensure-ascii-metrics-index

Handle UTF-8 data in metrics views