OpenSystemsDevelopment / qpdf

The Apache License version 2.0 is now the primary license for qpdf.
However, users may, at their option, continue to use Artistic version
2.0.

Preparing to refactor some pattern searching code to use these instead
of their own memchr loops. This should simplify the code that replaces
PCRE.

Also add copyright notice to a few public headers that were missing
one.

InputSource, FileInputSource, and BufferInputSource are now top-level
classes instead of privately nested inside QPDF.