Add the option to use local CSS and JavaScript minifiers instead of web services

Created by: pmetras

Issue https://github.com/paolobenve/myphotoshare/issues/60

This version changes the js-css-minify.sh script to let the user decide which minifiers to use. I've added the new options minify_css and minify_js in the myphotoshare.conf.defaults file where the user can select to use the default web services (options value web_service) or the name of a local minifier. For the moment, only cssmin, jsmin2 and jsmin3 Python scripts that are available as Debian/Ubuntu packages are supported, but new ones can easily be added in the js-css-minify.sh script. The resulting minified files from using cssminand jsmin3 are larger than when using the web services, but the minifying operation takes less time and is less prone to timeouts.