add indent support to `to_json` method
Sometimes you just want to have a quick look at your data in IPython and having `to_json` to support `indent` (as both `json.dumps` and `simplejson.dumps` do) would be very handy.

A workaround, which however doesn't always work, is to do `json.dumps(series.to_dict())`, which unfortunately is annoying when you don't have a JSON-serializable type (e.g., `numpy.bool_`).

I'm leaving this here as a food for discussion.
