Add SXXDL for managing smart unique_ptr implementations relating to SDL2
Update README.md to better describe the project, with appropriate anchor
links and a list of intended features.
Add ErrorBase to unify error handling across APG classes.
Update gitignore to ignore temporary markdown files.
Abstract tiled rendering out into a separate compilation unit.
Add some commonly used typedefs
Add a wrapper for "SDL" tilesets.
Remove some testing console output cruft.