PDFBOX-1513

Issue that depend to the changes
https://issues.apache.org/jira/browse/PDFBOX-1513

Changelog: 
- add ability to sign documents with xref streams (big thanks to A. Funk). This will significantly improve the signature creation, due to the fact that there are more and more documents with xref streams out there. 
- add ability to handle documents with hybrid xref (xref stream and table as fallback) 
- parsing incremental updated documents where the offsets of the xref entries doesn't match exactly the position of the object. The conflict solver tollerate objects that are +- 4 bytes out of the offset. 
- fix COSString parsing if there are malformed chars inside a hex string. 
- removed some confusing logging in the conflict solver 
- add ability to create and sign signature fields. 
- add ability to create pades signature timestamps (PAdES Part4) 
- improved the signature search and added new convenience methods to PDDocument. 
- add new methods to the PDSignature object (seed value dict) 
- add examples for signing pdf documents (just basic signatures, i try to add some for advanced signing with SignatureOptions and maybe a example for visual signing)