Definitions Folder

This pull request redirects the Custom Node Manager to use the DynamoPaths.Definitions folder as its default custom node search path. It updates the DynamoPaths.SetupDynamoPathsCore method to set the definitions folder to be in the user's AppData\Roaming folder. And it creates the packages and definitions directories, if they don't already exist, at the time of setup.

Yo @pboyer. Check this out! 
What happens if this fails to get a directory?  We should make sure we do not propagate a non-existent directory.  Again, this might happen if the user has not installed Dynamo, but is using it in a development context.