PHP Couchbase Extension 2.4.0

API documentation:
http://docs.couchbase.com/sdk-api/couchbase-php-client-2.4.0/namespaces/Couchbase.html

• Add more examples (XATTRs and RBAC user management).
• PCBC-505: Enable error maps for libcouchbase 2.8.0. For higher versions it will be enabled by libcouchbase.
• PCBC-499: Expose more query options on N1QL query for advanced tuning query performance
• PCBC-485: Expose enhanced error message. The server might fill properties "context" and "ref" of the error.