Replace gtest with sytem version

- Add new FindGtest.cmake to find installed versions.
- Add libgtest-dev for Travis
Merge pull request #731 from jspricke/gtest_removal

Replace gtest with sytem version (closes #717)
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.
Merge pull request #739 from georgebrindeiro/patch-1

Corrected elapsed time console print in line 73
Fix #740: Segmentation fault when "r" is pressed

Segmentation fault when "r" is pressed in PCLVisualizer.
The previous pull request #703 does not handle the "r" keyboard event
correctly and pass the event to vtkInteractorStyleRubberBandPick,
which enables rubber-band selection mode when 'r' is pressed.
Merge pull request #742 from xiangxw/fix_pclvisualizer_r_pressed

Fix #740 and #732: Segmentation fault when "r" is pressed
Better message printing for pcd_viewer

Camera file restore message will print after the file is loaded.
Merge pull request #744 from xiangxw/pcd_viewer_print_auto_restore_message

Better message printing for pcd_viewer
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
Fix unused parameter warning
Merge pull request #747 from jspricke/fix_unused_parameter

Fix unused parameter warning
Use "--amend" and upload tutorials/advanced to GitHub
Merge pull request #749 from taketwo/update-travis

Update documentation generation script for Travis
Add a separate "git add --all" command to Travis script

The command "git commit --all --amend" does not add untracked files, so they have to be added with a separate command.