Add support for docx xml containing a BOM

XML files within the DOCX file may begin with a byte order mark. Node doesn't strip this out automatically when reading UTF-8 data as a string, despite setting UTF-8 encoding ( joyent/node#1918 & joyent/node#4039 ). When these strings are passed to sax, they will cause errors because the XML string will not begin with the XML declaration or a node ("Non-whitespace before first tag.").