Zemeroth v0.5: ggez, WASM, itch.io, visuals, AI, campaign, and tests

Devlog post: https://ozkriff.github.io/2019-05-13--devlog-zemeroth-v0-5/

Main features of this release are:

- migration to ggez

- web version
- itch.io  page
- campaign mode
- AI improvements
- visual updates
- tests