Memoize the checking of resources on hosts to save on network requests

[mandatoryprogrammer]

sonar should never make two requests for a resource on a host. It should keep a running table for resources that it knows exist/doesn't exist and then check the table before potentially wasting time with another network operation.

For example:

var resource_table = {
    '192.168.1.2': ['/resource.css', '/resource.png', '/resource.jpg' ],
    '192.168.1.3': ['/resource.css', '/other.png', '/example.jpg' ],
}