Add Landscape config and badge

Landscape is working now so here's a badge and a config file to be getting started with. We can tailor it as needed. Currently the strictness is set to `medium`, but we can upgrade to `high` or `veryhigh`.

`medium`: 75%
106 errors
765 smells
293 style
https://landscape.io/github/hugovk/Pillow/78

`high`: 51%
191 errors
1184 smells
4781 style
https://landscape.io/github/hugovk/Pillow/80

`veryhigh`: 48%
219 errors
1218 smells
5262 style
https://landscape.io/github/hugovk/Pillow/81

Closes #895.