Allow BillboardSet / ParticleEmitter to choose whether billboards should face the camera or follow the node orientation. Closes #22.
Initial SDL 2.0.1 merge. Windows tested OK. Android will crash when returning to the application.
Fixed 07_Billboards compilation issue.
Fixed Android GL context destroy / restore. Removed project.properties file, as it's generated by the command "android update project".
Fixed exiting on Android.
Fixed black window on subsequent application starts on Android. Removed GL synchronization before swapping buffers, which should be unnecessary.
Refactor CMake build scripts.

Changes:
- Avoid globbing engine source files twice.
- Setup folder group for VS and Xcode. Also group object files according to 3rd-party library name.
- New define_source_files() macro.
Merge branch 'master' into sdl-update
Fixed Raspberry Pi error (could open Window).
Added option to OgreImporter to save material list file next to the model.
Fixed event handling for Raspberry Pi.
Updated documentation on additional Linux software package for RPI.
Reenable the function for RPI since new video driver support mouse.
Fixed mouse motion spin bug.

This bug only manifests itself when the SDL video driver has "WarpMouse" function implemented, like RPI video driver, in conjuction with call to SDL_SendMouseMotion() with relative event position.
Added ApplyMaterialList() function to StaticModel so that the feature can be used outside the editor as well. Refactored editor to use that function instead of its own implementation.
Unified the switch to output material list to -l in both AssetImporter & OgreImporter.
Removed the "use local IDs" toggle from editor as that can be specified with the import options.
Fixed runtime issue on Raspberry Pi with Pidora as OS.
Revert back to no-ops for Input::SetMouseVisible() on RPI.

The OS cursor on Raspberry Pi does not behave like other OS cursor on other platforms.
Refactor define_source_files macro.

Change the argument names to indicate their intent usage better. Add option to pass in directly globbing patterns.
Reverted back EXTRA_FLAGS which was changed unintendedly.
Unify how command line options are shown in utilities.
Added note in documentation about lack of space between command line option and its parameter.
Prepend Materials/ to the material list written by OgreImporter (assume "standard" materials subdirectory.)