Fix a minor bug regarding default lang for loaded texts

Also fix various small issues and undefined index
Prevent UTF conversion when not mandatory (post 0.7)
Try to delete picture from disk even if it has not been found in the database
Add basic charts; refs #157

Based on jquery jqplot library, add some charts:
- members status (pie),
- members state of dues (pie),
- contributions types (pie),
- contributions amount per month (bar).