Improve game manifest files

Fixes #4 by calling process.chdir(cwd) if cwd is set in `lexitron.json`

Implements scheme to fix #6, but with a few notes:
- `images` properties instead of `screenshots`, since they may be any kind of game related image
```
{
 "name": "Game Name", // required
 "description": "Game description. There will be a lot of space for this, depending on the image behind it", //optional

 "byline": "<Author information>", // optional
 "website": "<Hostname>", // optional
 "twitter": "cardboardcompy", // optional

 "images": <Array of image paths>, // requires at least 1
 "videos": <Array of video paths>, // optional, not currently used
 "players_min": <Minimum players>, // optional, not currently used
 "players_max": <Maximum players>, // optional, not currently used
 "controls": [ // optional, not currently used
  {
   "player": 1,
   "<Button name>": "<Button description>"
  }
 ],

 "exec": "/home/lexitron/.steam/steam/SteamApps/common/KentuckyRouteZero/KentuckyRouteZero", // required
 "args": "", // optional
 "cwd": "/home/lexitron/.steam/steam/SteamApps/common/KentuckyRouteZero/" //optional
}
```

Does a little more for #1, but the layout is still very basic and the slideshow doesn't seem to work