Implement API connection test via Test-SlackApi

[hmmwhatsthisdo]

Resolves #1.

Once #53 is merged in, augmenting this to take advantage of the additional error output should be very straightforward.

I'm not sure if we want to write a Pester test for checking the API's availability from inside of AppVeyor (it would cause builds to fail if the Slack API is having issues), but it's at least a thought.

[RamblingCookieMonster]

Looks good to me, apart from a silly nitpick: Would you mind moving the help into the function body?

Tests against Slack from AppVeyor make sense to me; would probably just want to be judicious with their use - with the rate limiting you added, I suppose we should be good : )

We could certainly serialize expected results/errors/etc, but having tests that hit the actual API might catch breaking changes to the API - if there are Slack issues during a test, I could always just re-run the build

Cheers!

[hmmwhatsthisdo]

Would you mind moving the help into the function body?

Whoops! Didn't notice I had put it outside the body on that one. It's been fixed.

Now that API error handling has been merged in, I should be able to apply it here without much trouble. I should have something pushed within the next hour or two.

[hmmwhatsthisdo]

API error handling works, but while testing things out I realized there's a minor bug with the error-handler code in Send-SlackAPI: failures that occur below the API level (i.e. slack.com being un-resolvable due to not having Internet access) will throw an exception when trying to parse the (non-existent) error message. I'll make an issue/PR for this shortly. See #55/#56

[RamblingCookieMonster]

Awesome, thanks! Merging