Include deprecation warnings for JSON components.

This will further aid in the removal of all-things-JSON for the
future.
Skip exposition of Summary children w/o samples.

Issue https://github.com/prometheus/client_java/issues/27 reported
a case whereby the quantile estimation library would throw an
IllegalStateException for the request of quantile ranks for observation
streams that are empty.  This was a fix for bad upstream behavior of
original estimator library that would either throw a
NullPointerException or return a zero sentinel value depending upon
what condition happened under the hood.  Nevertheless, the original
fix was insufficient for the client library.

What has been done is provide a filter mechanism in exposition to
prevent serialization of empty Summary metrics.  Along with this
has been an upgrade of the quantile estimator library so it no
longer throws an exception.
Merge pull request #29 from prometheus/refactor/summary/observation-value-buffering

Fix Empty Sample Error