Changes:

• using Bootstrap for HTML styling
• code cleanup in AdminClass.php:
  • most of UI tasks are now implemented client-side
  • consolidation of method results as associative arrays
  • redundant and obsolete code removed
• separation of editing and removal in different php files
• fixed several bugs in the data model, e.g., updating a group's gid also updates the gids of the users
• support for sqlite3 database
• support for pbkdf2 hash algorithm

Thanks to @ricardopadilha for this release