CMake Configuration Improvements

work on #82

- Added the option to use and existing download of entityx rather than
downloading it every time.
- Had LibGunship build as static so it can be linked against external
projects.
- Added the option to not build the default executable, useful for
projects defining their own entry point.
- Added Engine::ResetCurrentScene() to provide a way to reset the scene
without giving an initialization function.
- Removed example code from Gunship.cpp since we are now setup up to be
able to have Spectacle as an external project.