Use kindlestrip to remove redundant data generated by KindleGen

Kindlegen 1.1 and later now always adds the source files used in
compiling the kindle ebook as one of the (invisible) records in the
kindle ebook, making the generated one bloated.

Import kindlestrip from http://www.mobileread.com/forums/showthread.php?t=96903
to strip this data. (Thanks byelims for the tip!)