Added script to allow themes to be imported via command line

With this script, the following deploy process should be possible:

1. Make local changes to styles and templates
2. Use "Export theme" in myBB to create a new template file (Starbow-theme.xml) to update the file in /themes
3. After changes have been pulled from the respository, run import_themes.php on the command line to generate new .css from the template file.