Add contributors file and add all contributors to about dialog

[kliment]

It's about time to credit all the contributors to the Printrun project more prominently. I've added a CONTRIBUTORS.md file linkedf from README and added the list there into the credits dialog as well. Comments very welcome, also on whether there's a better way to format it.

[rockstorm101]

Hi @kliment, thank you for you effort on compiling this list. Much appreciated. My only comment would be that keeping two virtually identical lists is usually a pain and there's a high chance they get out of sync. Could we somehow make the list in the 'About' page to emanate from the one in contributors.md? Something along these lines in pronterface.py:

for each developer in contributors.md do
    info.AddDeveloper(developer)

This would also make pronterface.py less cluttered.

[kliment]

If you know a way to do it, sure. I just didn't want to implement a markdown parser...

[rockstorm101]

Fair enough, I gave it some thinking and I couldn't figure out how to do it so I'm good with this as is.