upgrade Net Framework to 4.5 and improved folder dialog

[pedoc]

replace System.Windows.Forms.FolderBrowserDialog to Ookii.Dialogs.WinForms.VistaFolderBrowserDialog for better user experience

Please fill out the following checklist:

• [ ] Added tests for any bug fixes that changed existing code
• [ ] Added tests for any new behavior
• [x] All unit tests are passing (please make sure all tests are passing for your branch/PR at https://ci.appveyor.com/project/activescott/lessmsi/history )

If you need any help at all, feel free to submit the PR and @-mention activescott and I'll be happy to assist!

[activescott]

Thanks for the proposal @pedoc but I'm not interested in updating to 4.5 nor to take on another dependency.