ZF-4872: Add core support for Google Data protocol version 2
Add core support for Google Data protocol version 2, including support for the final AtomPub specification as defined in RFC 5023.
- Support for the new 'GData-Version' HTTP header.
- New namespaces for AtomPub (app) and OpenSearch (openSearch) in version 2.
- Support for using ETags to implement optimistic concurrency (ETag headers and gd:etag XML elements are recognized, and later used to set 'If-Match' HTTP headers).
- Support for setting arbitrary headers on most HTTP-bound operations.