Make the dtproj.user file optional

[fmms]

Usually it is best practice not to commit the *.dtproj.user file as it is user specific and just personal preferences (see https://stackoverflow.com/questions/7756620/what-files-should-i-check-in-for-an-ssis-project and https://docs.microsoft.com/en-us/sql/integration-services/integration-services-ssis-projects-and-solutions)

This solves #14.