flux-kvs usability improvements

Made the following improvements:

* Added `flux-kvs type key [key...]` which prints the type of a key
* Changed `flux-kvs get` and `flux-kvs dir` to look at the type of the returned object, and run it through the appropriate `json_object_get_<type>()` before display.  This will de-escape JSON strings, for example.
* Strings are printed without quotes (updated tests accordingly)
* `flux-kvs dir` no longer requires an argument.  Without one, the root directory is listed.