#17 ACL helper methods

[Jeremy Dorn]

Add an ACL class with some helper methods and use them throughout the site. The methods are as follows:

• canEditPage($page)
• canDeletePage($page)
• canEditElement($page,$element)
• canDeleteElement($page,$element)
• canBackup()
• canRestore()
• canDeleteElements()
• canDeletePages()
• canEditPages()
• canCreateElements()
• canCreatePages()