This allows us:
- dogfood browser contexts the way we want them to be used
- simplifies the dance around service workers / cookies setting up and tier down.
This patch introduces Browser Contexts and methods to manage them:
- `browser.createIncognitoBrowserContext()` - to create new incognito
context
- `browser.browserContext()` - to get all existing contexts
- `browserContext.dispose()` - to dispose incognito context.
Fixes#85.