improve cmake (portable on all OSes), add appveyor and travis CI

[cenit]

Plus other minor fixes

[AlexeyAB]

@cenit Thanks! Yes, it seems it's better to make uniform short paths for both Win/Linux. Travis and appveyor are good things too. I will try to test it when I will have a time.

[cenit]

sure, no hurry. I just wanted to share what I had to do on this project to adapt to my workflow. If there's anything interesting for you, please feel free to pick it :)

[AlexeyAB]

Thank you so much for all your work!