building.markdown: remove dependency install info

Too much effort to keep these up to date for all supported platforms.
Clean up Markdown documents
readme.markdown: add link to build instructions
readme.markdown: update screenshot
readme.markdown: link to license.markdown
effect: use map instead of unordered_map

This removes the need to specify a custom hash implementation for
std::pair<std::string, std::string>.
assemblages: don't repeat default parameters
Improve class names

components::model → components::model_renderer
systems::model_system → systems::model_renderer
systems::render_system → systems::renderer