libppd 2.1b1

First beta release for libppd 2.1.0.

New features (more to come):

• Support for building with libcups3, CUPS library of CUPS 3.x
  (Pull request #27)
• Convert INSTALL to INSTALL.md
  (Pull request #34)

Bug fixes:

• Prefer PDF again in PPDs for driverless printers
  PDF works better with finishing, especially combinations of multiple copies, collation, and stapling/binding (Issue #42, Pull request #44).
• Use 0.5mm as tolerance when comparing page sizes
  For the PWG two page sizes are considered the same when the dimensions differ no more than 0.5 mm, libppd used too tight tolerances (Issue #29, Pull request #46).
• PPD generator: Check for required attributes when choosing input format
  Check for PCLm and PWG the minimum of attributes which we require during PPD generation. (Pull request #39, #40, Fedora issue).
• ppdLoadAttributes(): Improve check whether parameters are integer
  (Pull request #38)
• ppdLoadAttributes(): Fix crash when page size could not get determined
  (Issue #31, Pull request #37)
• Fix crash if there is no page size for "Custom"
  (Pull request #35, OpenPrinting/cups#849)
• Fix crash when incoming *ptr is NULL
  (Pull request #28, same as OpenPrinting/cups#831)
• libcups2 compatibility: Use proper CUPS array callback function types
  Fixed CUPS array function call in libcups2 compatibility layer (Pull request #33)
• Build system: Fix failure to correctly link to zlib
  Look up zlib properly with pkg-config (Pull request #32)