#75 incorrect img test regarding folders

[mkarlsson]

If you have a book in a folder e.g. called "C:\books\de(mo)" and the book itself contains some images located in a sub folder, the hasValidRelativePath() call for the ImageTests class fails and says it contains a incorrect path.
ZedVal shouldn't bother about the folder above the book itself - it should only validate the folders relative to the OPF.