Initial release

This release is derived from PSR-7, however many significant changes have been made. Highlights include:

• all implementation-defined objects have been removed; if these are required, an is check is expected. This removes the need for mixed types on methods such as getParsedBody()
• similarly, removed getAttribute
• StreamInterface has been removed, in favor of HH\Lib\Experimental\IO\ReadHandle and WriteHandle
• This means that Message does not expose the body - requests have a read handle, responses have a write handle
• replace ?string with string where there is not a meaningful distinction (in HTTP) between 'empty string' and 'not provided'
• form variables are accessible via flat dict<string, string> structures, not arbitrarily nested trees