Replace Step Functions with CloudWatch Events

[toricls]

This PR closes #6.

• Removes unnecessary Lambda functions: build-dispatcher, build-result-exporter
• Removes the Step Functions resource defs from sam.yml (it cannot be removed due to the CFn's buggy behavior, see the comment in sam.yml)
• Rename Lambda function build-result-notifier to build-state-notifier because it notifies not only success/failure but also in-progress
• Creates a CloudWatch Events resource to invoke build-result-notifier to update GitHub status by CodeBuild project's status changing
• And some cosmetic changes in sam.yml