Add diagnostic data dll names to .gitignore

These dll's are used for reporting diagnostic data and users can
optionally store them in the module's directory to avoid downloading
them on demand (or they can use Set-GitHubConfiguration -AssemblyPath
to specify an alternate location).

For users that opt to put the dll's in the module directory and are using
the module from a git enlistment, this will ignore those files.

This should be temporary and should be able to be undone once microsoft#104 is resolved.

Author: HowardWolosky

.gitignore

@@ -1 +1,4 @@
 Tests/Config/Settings.ps1
+Microsoft.ApplicationInsights.dll
+Microsoft.Diagnostics.Tracing.EventSource.dll
+Microsoft.Threading.Tasks.dll