Appveyor plus cmake enhancements

[rodneyp290]

This pull request adds Appveyor CI to the project (addressing #39), and makes some improvements to the previous CMake pull request #37. Successful builds can be seen here

CMake Improvements include:

• enabling the importing of OpenDIS6 and/or OpenDIS7 lib into other CMake projects.
  • I've also added a separate CMakeList.txt to the example directory that can be used to test installations via importing OpenDIS6
  • in doing this I altered the example c++ file includes to use the relative includes (eg. #include "file.h" instead of #include <example/file.h>)
• adding a if statement for defining SDL libs to include, and SDL2main (required by Windows) broke the build on some Linux distributions
• using GNUInstallDir module for better handling of lib install directories on some Linux distributions

[rodneyp290]

Just added the last commit as the related issue #21 seemed like a fairly simple issue

[leif81]

Looks great @rodneyp290 !