Generate addins directory.

This pull request fixes #1899. For some installations, the addins directory specified by the Revit addin utility will not exist. Previously this occurred when a version of Revit had been installed, but not run. This change, checks for the specified directory and generates it if neccessary. 

There is one potential problem here. The Revit team has recently noticed that the addin utility will return "Unknown" for the addin directory if they haven't yet added the revit version to the table of versions that the addin utility uses to generate the directory name. The worst case is that we generate a directory called "Unknown", and later generate an appropriately named directory when the addin utility is updated.

For @lukechurch to review.
That's actually just to silence a Resharper warning that wants you to be sure that dir != null before you use it, even though we're using Directory.Exists() which does the job.