Reader API

This adds wrappers around the [Reader API](http://www.xmlsoft.org/html/libxml-xmlreader.html)

Example:

    reader := golibxml.ReaderForMemory(xml, "", "utf-8", golibxml.XML_PARSE_NOENT)
    defer reader.Free()
    for parser.Read() == 1 {
        fmt.Printf("%v: %v\n", reader.Name(), reader.NodeType())
    }