Fix WiX warnings

@speth I made some changes to the way the registry keys are created and deleted in the wxsgen.py file, because WiX was complaining about the @Action key being deprecated: http://wixtoolset.org/documentation/manual/v3/xsd/wix/registrykey.html Please let me know if these changes break anything, or if there's a different way you'd prefer to handle the options (for instance, combining the dictionary creation into reg_options, so that only that is passed into addRegistryKey)!

Thanks!
Bryan