Support excluding 2D functionality from the build (CMake option URHO3D_URHO2D=0). Removed empty scene creation from the UIDrag example. Make compiling Civetweb conditional of URHO3D_NETWORK. Exclude samples depending on the Controls structure when URHO3D_NETWORK is disabled.
More robust check for OcclusionBuffer culling mode. Now also CULL_NONE mode should detect triangle facing correctly.
For Travis CI - switch CI build to use OS X build environment.