* remove pass by reference for php setters

* comments out memory leak test

* Add json encode/decode for php.

* Fix php conformance test on 32-bit machines.

* Fix conformance test for c extension.

* Fix comments

1. Generate single file for each message.
2. Lazily initiate metadata.

This pull request includes two implementation: C extension and PHP
package. Both implementations support encode/decode of singular,
repeated and map fields.