Doxygen filter for Google Protocol Buffers .proto files.
====================
How to enable this filter in Doxygen:
  1. Generate Doxygen configuration file with command 'doxygen -g <filename>'
       e.g.  doxygen -g doxyfile
  2. In the Doxygen configuration file, find JAVADOC_AUTOBRIEF and set it enabled
       JAVADOC_AUTOBRIEF      = YES
     This is required since version 0.6-beta
  3. In the Doxygen configuration file, find FILE_PATTERNS and add *.proto
       FILE_PATTERNS          = *.proto
  4. In the Doxygen configuration file, find EXTENSION_MAPPING and add proto=C
       EXTENSION_MAPPING      = proto=C
  5. In the Doxygen configuration file, find INPUT_FILTER and add this script
       INPUT_FILTER           = "python proto2cpp.py"
  6. Run Doxygen with the modified configuration
       doxygen doxyfile
====================
Version history:
--------------------
 0.1-pre-alpha (2012-06-13)
  - initial version
--------------------
 0.2-pre-alpha (2012-06-15)
  - added support for enums
--------------------
 0.3-alpha (2013-01-29)
  - moved .proto file parsing logic to another function
  - added comments to the file
--------------------
 0.4-beta (2013-08-29)
  - 'classified' proto2cpp and updated documentation to make the
     script itself Doxygen compatible
  - changed all print statements to print() functions
     * 64-bit Python v3.3.1 running on 64-bit Windows 7 Home Premium
       did not automatically convert print statements to print()
       functions but instead raised a syntax error
  - made a change so that .proto files are converted before printing
    and other files are printed to stdout as is
     * this allows using the filter with multiple file types
--------------------
 0.5-beta (2014-11-16)
  - fixed enum ending to have semicolon to have proper enum syntax
    in struct (thanks to m47iast for pointing this out)
--------------------
 0.6-beta (2015-07-27)
  - made output to be more compact by removing extra empty lines and
    not moving member comments before the member but keeping it after
    the member instead
     * these changes lead into need of enabling JAVADOC_AUTOBRIEF
  - added steps for enabling the filter in Doxygen in this file