Add support for signaturesFilelist and signaturesFile elements

[GoogleCodeExporter]

You should be able to enforce specific signatures files, to *enforce* specific 
files. Filesets are too relaxed, because they simply ignore missing files, 
leading to no checks done.

This allows to do the following:

<forbiddenapis>
 <signaturesFile file="sign.txt"/>
 <siganturesFilelist dir="." files="file1.txt, file2.txt"/>
<forbiddenapis>

Original issue reported on code.google.com by uwe.h.schindler on 14 Sep 2014 at 9:31