Implement visible digital signatures

[jonathanj]

This seems like a good starting point: https://svn.apache.org/repos/asf/pdfbox/trunk/examples/src/main/java/org/apache/pdfbox/examples/signature/CreateSignature.java

[jonathanj]

This might also be useful for "visible" signatures: https://svn.apache.org/repos/asf/pdfbox/trunk/examples/src/main/java/org/apache/pdfbox/examples/signature/CreateVisibleSignature.java