WebStat: minor improvements

• When renaming a column in webstat.cfg and running ./webstatadmin --load-config, the column was deleted from the database. Now, the user is asked if he wants to delete it or rename it (fixes #546)
• Change line graphs to bar graphs
• Add average, max and min values for events
• Fix total records count now also counts restricted collections
• Clean up code
• Improve display of data when selecting last 2, 5 or 10 years
• Change names of bibcirculation stats