Filter Networks by attrs and more cleanup

Added ability to filter networks by attributes.
    * Removed specific handler for this
    * Added as options to general networks handler

Moved networks method as an instance method on Site rather
    than a class method under Network since you'd always have
    to pass in a site and session anyways.
Most handlers weren't 404ing for invalid site. Now they are.
Added description field to attributes
Name is now immutable for attributes