Continue adding code to parse xref tables for existing PDF files.
Commit of ongoing work to read full xref trail in pre-existing files.
Separate readTrailer() into a separate function and perform minor cleanup.
Record the most recently written pre-existing trailer while reading the previously written xref tables.
Clean up use of xref-entry "dirty" flag in file.go and add a file-level "dirty" flag.
Minor cleanup.
Move all modification of xrefEntry into setters.  Simplify parameters of file.DeletObject().
Allow trailer to be rewritten on a pre-existing PDF file as an update.
Also, reorder exported methods in pdf/file.go
Move main() for integration tests into its own file.
Rename "tests" directory to "integration_tests".
Remove several statements that were inserted for debugging and an out-of-date comment.