PSR http Message Files

This tag contains all updates since entering the second review of PSR-7. These include: - Clarifications to RequestInterface::withUri() specifying that changes MUST also update the Host header unless the $preserverHost flag is true. - Removal of the various getHeader*() method overrides in RequestInterface, as they are irrelevant with the above change. - Added verbiage to the RequestInterface class-level docblock indicating that constructors SHOULD attempt to set the Host header in order to ensure the message is valid on initialization. - RequestInterface::getHeaderLine() MUST return a string (null was allowed previously, which leads to ambiguity of return value). - ResponseInterface::getReasonPhrase() MUST return a string(null was allowed previously, which leads to ambiguity of return value). - ResponseInterface::withStatus() now sets the default value of the $reasonPhrase parameter to an empty string (to correspond to the above change). - UploadedFileInterface::move() was renamed to moveTo(), and the argument renamed to $targetPath; this clarifies intent for the method. - UriInterface::withPath() has clarifications in the docblock to indicate the four possible path types (empty, root path, relative path, domain-relative — or absolute — path).