Automated tests

I'm currently making some integration tests using Spectron and mocha so we can do automated test on builds before uploading the releases (this would catch errors like the one in #30 before it's actually uploaded). Generally this could save us a lot of time doing manual testing and fixing bugs that should have been caught before release.

Please let me know what features that would be nice to have tested. So far:
- [x] Able to startup (https://github.com/inkle/inky/pull/35)
- [ ] Able to write text in editor
- [ ] Able to save text in editor
- [ ] Ability to compile a minimal story. This is probably defined as: when text in editor is changed, then the player view refreshes with the expected content?
- [ ] Ability to export to JSON (where the JSON has valid content - probably no need to test correctness, since JSON format may change over time, and that might be annoying to update)
- [ ] Ability to play through a more complex story to the end. (not sure how easy this is to set up)