Slack Integration

[adamfisher]

Closes #26

A few things to note or consider:

• I had to pull in pieces of the pending Azure Blob storage branch to get a consistent foundation for the solution and ancillary config files so I just deleted those related to specifically to Blob storage from my branch. I know this won't be the normal thing we need to do in the future.
• I really would like to make the inputs to activities more rich and dynamic but unsure how to achieve that. I'm also wondering how time-consuming that will be (e.g. for a channel input you could populate a dropdown of available channels based on what can be found for the Slack instance you are connected to).
• Most of the event activities are not implemented yet because I'm unsure of how to approach making those triggering activities using Elsa's built-in semantics (e.g. The WatchDirectMessages acts as a trigger activity and waits for a new slack message to be published to a channel for the workflow to start).
• How do we want to approach testing in general - unit or integration testing? I am leaning toward unit testing only and no integration testing due to how many integrations we may get over time and setting all of them up and running them against lots of disparate services seems daunting especially for a new developer that wants to work on integrations. So pulling in Moq or NSubstitute and using Bogus.Faker to generate fake responses for these services and doing only unit testing will allow us to scale nicely as we add more services.
• I'm not a fan of doc comments being enforced in the test project as it seems overly wordy to me.

[sfmskywalker]

I think this is a great starting point. Regarding your comment on automated tests, I agree that we should start with unit tests and leave the end-to-end tests be, for now. As for XML documentation, I hear you, but we will probably be adding it anyway because it does help others to understand how things are to be used and fit together (when written properly). That said, I won't require it for this PR to go in.

Thank you for this awesome PR!