Add option to also ignore unresolvable signatures in Ant and CLI

[GoogleCodeExporter]

In issue #14 support was added to Maven Mojo, so the build does not fail on 
unresolvable signatures. This was not added to Apache Ant, but it may be useful 
as workaround for issues like issue #41.

This is an easy change, just ad setter to Task and pass this to Checker ctor 
(currently always "true" is passed, to fail).

Original issue reported on code.google.com by uwe.h.schindler on 23 Nov 2014 at 12:01