Master

Added GUI widgets and model-viewing app
Fixes travis for race-condition on macOS
Fixes appveyor configuration and to build all branches
Updated travis.yml to support Ubuntu 18.04, gcc-7, and clang-7.0
Contributors guidelines updated
Avoid cstdlib random generators in ransac registration, use C++11 random instead.
Fixed a bug in open3d::geometry::TriangleMesh::ClusterConnectedTriangles.
Added option BUILD_BENCHMARKS for building microbenchmarks

0.9.0