Added parser class to convert streams of tokens to streams of PDF objects.

Added nothing variant type to distinguish between a PDF null object and "no object".
This was necessary because std::optional is not yet part of the C++ standard.
Added object reference type to variant.
atom_table should be passed by reference to parser constructor.
Renamed lexer.cpp/hpp to parser.cpp/hpp.
Added global (to pdf namespace) pdf_atoms initializer table.
Added lldb to vagrant vm.
Might as well add gdb too.
Fixed pdf_atoms initialization.