Camera implementation for Linux

Based on an ad-hoc camera app I made: https://github.com/Davideddu/kivy-camera
Kivy Camera needs to be installed system-wide, as the plyer module will look for its wrapper in the path.

It just runs the camera app using subprocess, then calls the callback.

I decided to make a new camera app because the existing one didn't have a feature that plyer requires, which is taking a picture, saving it to a specific path and exiting. The app should also work as a normal camera app (I didn't test it).

This should work on Windows and Mac OS X too, however kivy-camera's makefile would need some fixes in order to put the files in the right places in those platforms. Please pull request it.