OpenSystemsDevelopment / qpdf

Make some more methods in QPDFPageObjectHelper work with form
XObjects, provide forEach methods to walk through nested form
XObjects, possibly recursively. This should make it easier to work
with form XObjects from user code.

Turns out unreadCh is much more efficient than seek(-1, SEEK_CUR).
Update comments and code to reflect this.

Make clear that you almost always want + or - before an angle when
specifying rotation.

Issue #399 mentioned a use case for which qpdf has support, but the
fact that it is supported was not documented or in the test suite,
making it vulerable to accidental breakage.

Allow exit status-based checking of whether a file is encrypted or
requires a password without necessarily supplying the correct
password. Useful for scripting.