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)
model outlier removal contribution
Code upate
Signed-off-by: Victor Lamoine <victor.lamoine@gmail.com>
Update tutorial
Remove model_from_normals_ class member
Use pcl::isFinite instead of pcl_isfinite