Make the PHP generator use non-hardcoded namespaces.

Make the PHP generator use non-hardcoded namespaces.

- Make the PHP generator use program->get_namespace("php")
  instead of program->get_php_namespace()
- Eliminate the explicit "php_namespace" in t_program.
- Deprecate the php_namespace token.
- Update example .thrift files and syntax files.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@745241 13f79535-47bb-0310-9956-ffa450edef68