4.3.0: AWS service endpoints

• Merged PR #20: Add Environment::endpoint() to return AWS service
  endpoints
  (@thekid)
• Merged PR #19: Extract AWS core library - containing credentials
  and AWS service endpoints, see https://github.com/xp-forge/aws
• Merged PR #18: Make credentials accessible via credentials() in
  Environment (instead of having to use AWS_ACCESS_KEY_ID/...
  environment variables).
  (@thekid)
• Merged PR #17: Migrate to new testing library - @thekid