* Add well known types to php runtime.

* Fix php7.0 tests

* No longer generate empty.proto in test as it has been included in
runtime.

* Fix zts build

* Clean code

* Rename g_p_b_empty to empty.

* Don't generate code for empty.proto in compatibility test

* Fix 32-bit

* Fix mac build

* Fix Makefile.am to add new files

* adds PHPDoc @return and @param for getters and setters respectively

* addresses changes in PR review

* adds documentation tests

* Update php_generator:

- Prepend \ to names where required
- Remove <pre> tags
- Update protobuf field comments

* Updates class files with the protobuf changes

* Addresses review comments

* removes Protobuf Type line from PHP generated classes

* fixes phpdoc test

* adds array types to phpdoc

This option will be prepended to generated classes of all messages in
the containing file.

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