Replace gtest with sytem version

- Add new FindGtest.cmake to find installed versions.
- Add libgtest-dev for Travis
Corrected elapsed time console print in line 73

Corrected return value for watch.getTimeSeconds() from int to double. Also corrected message that said time was in milliseconds, when watch.getTimeSeconds() returns elapsed time in seconds.
Use a faster n-point sampler (thanks to Troels Bo Jørgensen for this code)
Remove a bug in a useless extra rejection step that caused a continue in all main loop iterations but the first (thanks to Jeppe Pedersen and Kent Hansen for catching this)
Use a cache for already computed feature correspondences (thanks to Jeppe Pedersen and Kent Hansen for the code)
Avoid duplicate squared Euclidean distance computation
Remove duplicate archive and use compressed PCDs to save space
Explicitly specify the required PCL modules in the CMakeLists.txt file
Update the tutorial code to reflect the optimizations, leading to much faster and more robust pose estimation