libppd 2.0b2

Second beta release approaching libppd 2.0.0: Support for more than 2 resolutions in Apple Raster/AirPrint, fixed crash when filter functions are called without supplying PPD file, adapted to libcupsfilters cfCatalog...() API correction, tons of fixes in source code documentation and build system

• PPD file generator for driverless printing with CUPS: Support more than 2 resolutions in Apple Raster/AirPrint. The urf-supported IPP attribute was only parsed correctly when its RS part had only 1 or 2 and not more resolutions specified. This commit corrects now for an arbitrary amount of resolutions, taking the lowest for "draft", the highest for "high" and one in the middle for "normal" print quality (PR #3).
• Update cfCatalogLoad() calls for API change in libcupsfilters. In libcupsfilters we have added language/translation support to the cfCatalog...() API functions via OpenPrinting/libcupsfilters#2. This changes the cfCatalogLoad() calls in libppd (both in the PPD generator for driverless printing). For a quick solution we supply NULL as language for now, resembling the old behavior. We look into language support in the PPD generator later (PR #4).
• ppdFilterEmitJCL(): Added NULL check for PPD not being supplied. Classic CUPS filters created based on filter functions using ppdFilterCUPSWrapper() and also filter functions of libppd (ppdFilter...()) should also work without PPD file and not crash if no PPD file is supplied.
• Make build of genstrings optional.
  genstrings is only a development tool for the PPD compiler, not a user tool, therefore we make its build optional. It can be built via make genstrings or the ./configure option --enable-genstrings.
• Makefile.am: Include NOTICE in distribution tarball
• configure.ac: Added foreign to to AM_INIT_AUTOMAKE() call. Makes automake not require a file named README.
• Cleaned up .gitignore
• Tons of fixes in the source code documentation: README.md, INSTALL, DEVELOPING.md, CONTRIBUTING.md, COPYING, NOTICE, ... Adapted to the libppd component, added links.