sass:watch fixes for windows and linux

Windows:
sass:watch raises an exception when using properties in source/destination (like ${project.build.directory}). I've converted separators to unix (like org.jasig.maven.plugin.sass.Resource#getDirectoriesAndDestinations already does).

Linux:
sass:watch reports following error:
Missing dependency 'rb-inotify' (version '~> 0.8.8')

I've added the rb-inotify gem to plugin.