added first pass Travis support file
CMake: make SFML an optional dependency
CMake: relax version requirements for ffmpeg libraries
separate SFML-dependent bits into tools

There were some old SFML-dependent bits in cldtcwt
add fallback SFML searching mechanism
make FFMPEG an optional dependency
use new-style OpenCV header
use std::chrono::system_clock
add test-suite to travis config
Merge in travis CI fixes

This branch integrates Travis-CI support and the required changes to
make the library buildable without V4L, FFMPEG or SFML if necessary.