PDFBOX-2512: OutOfMemory while signing large documents
- 1. clean the objectpool while closing the document
- 2. pass a BufferedInputStream to the SignatureInteface / remove some code
- pass a scratchfile to the load method in the example 'CreateSignature'

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/branches/1.8@1640774 13f79535-47bb-0310-9956-ffa450edef68