Introduce ELASTICA_JSON_UNESCAPED_UNICODE ELASTICA_JSON_UNESCAPED_SLASHES

Avoids using the global defines from PHP. Follows up #559.

This lead to https://bugzilla.wikimedia.org/show_bug.cgi?id=65110 being filed downstream by myself. We use the existence of JSON_UNESCAPED_* defines to detect 5.3/5.4 behavior and handle fallbacks more gracefully. Elastica unconditionally defining these on 5.3 and below breaks our behavior detection.

Proposed PR should work on 5.3 as well as 5.4+