sendtokindle : fix cover missing problem

add this line into xxx.opf (ex. content.opf)

 <item id="cover" href="PATH/cover.jpg" media-type="image/jpeg" properties="cover-image"/>

epub to Kindle, fix TOC missing problem

unzip epub file, 

1. check for opf file

<item href="toc.ncx" id="toc.ncx" media-type="application/x-dtbncx+xml"/>

<spine toc="ncx">

in these two tag, item with type x-dtbncx+xml, id must show in spine

<item href="toc.ncx" id="ncx" media-type="application/x-dtbncx+xml"/>

2. check for toc.ncx 

src="...", check its path, correct it from toc.ncx path

話說, Kindle format convertor 對於 epub 容錯性很低啊