Added check for display_admin_url config item

Added check for display_admin_url config item which if not 
set to 1 will prevent the server from revealing the admin URL
when the user visits the incorrect domain
(more useful in combination with the musthost config item)
added display_admin_url config to default config

The default value for this is 1 (to preserve backward
compatibility).
Inverted the comparison of display_admin_url

I've inverted the comparison of display_admin_url so 
that it must be set to 0 in order to disable it
removed default config item
changed display_admin_url to hide_admin_url

changed display_admin_url to hide_admin_url and 
changed comparator to test if this value is not set to 1
Merge pull request #175 from Perturbatio/develop

Added check for hide_admin_url config item