Refactored PdfObjectString into a struct with bool flag for hex. Fixed any code broken by the change.
Unexported non-essential functions for crypto (not used by model). Can unexport more later or refactor to internal package.
Previously Tm boxes would have their text extracted, but with no
formatting information between boxes. Now, if a box is lower on the
page than has previously been recorded, we insert a newline, and if
a box is further to the right on the same line than a previous box, we
insert a tab character.
This works well enough for parsing Clipper Card transaction histories.
You can see an example at https://clipper-csv.appspot.com.
Fixes#141.
