Using ts-jest for coverage

[sverhoeven]

Fixes #17

[jspaaks]

npm run test is currently failing, need to add squiggly braces around App import here:

create-react-app/packages/react-scripts/template/src/App.test.tsx

Line 4 in cb35524

import App from './App';

this is because of the linting rules we've added, which say that you cannot have default exports, hence you need to use { } when importing variables