THRIFT-2712 PHP: add "json" option to create JsonSerializable code

This patch adds a "json" option to PHP code generation that will
make generated classes implement JsonSerializable, so thrift
objects may be converted to json using json_encode() easily.
If the "validate" option is enabled, the object's write validator
will be called, beyond that the jsonSerialize() method only outputs
non-null fields in the JSON object (JSON parsers get grumpy if you
send them null where they expect to see a number).

Patch: Stig Bakken

Github Pull Request: This closes #219