use a submodule for the data

In short, `lib/domains/` was deleted, and a submodule pointing to https://github.com/leereilly/swot was added under `lib/data/`. The interesting changes:

```
$ git checkout data-submodule
$ git diff origin/master --name-only | grep -v '^lib/domains/'
.gitmodules
CONTRIBUTING.md
Rakefile
lib/data
lib/swot.rb
swot.gemspec
```

c/o http://stackoverflow.com/a/4381121/358804.

/cc https://github.com/leereilly/swot/issues/247