+[stringWithPath:] -> +[pathWithComponents:].

This removes -[initWithPath:,...], so that it's finally possible to use
-[initWithPath:] for many places where a workaround like
-[initWithDirectoryPath:] was required before.
Rename -[initWith*Path:] to -[initWithPath:].
Rename +[archiveWithFile:] to +[archiveWithPath:].
OFHTTPRequest: Add -[isEqual:] and -[hash].
OFHTTPRequest: Conform to OFCopying.
OFHTTPResponse: Include self in -[description].
OFZIPArchiveEntry: Add -[description].
Add OF_HAVE_CLASS_EXTENSIONS.
Move private methods into private headers.
OFZIPArchiveEntry: Export the extra field.