Reorganized directory; added basic unit tests; migrated to terraform-plugin-sdk

[isabelgiang]

• Migrated from hashicorp/terraform to hashicorp/terraform-plugin-sdk. This includes the acceptance test framework that is commonly used to test Terraform providers.
• Moved files into a slack/ repo to address Place resources and datasources on a different directory. #15
• Renamed files in slack/ to match conventions I found in other repos like kubernetes
• Added one basic unit test for datasource_slack_user and resource_slack_channel. This partially addresses Implement tests for resource/data sources #13

[TimDurward]

Great addition, thank you, I'll be reviewing this shortly!