started some refactoring and tidying of acts_as_asset

moved into its own module namespace, and split the instance methods up into related areas within their own modules
moved acts_as_isa into its own seek module namespace

in line with acts_as_asset
removed unnecessary require 'acts_as_asset' from models
also split off the associations (has_many etc) from acts_as_asset into the appropriate module
split off the acts_as_asset search class methods
removing what looks like an unused method
updated seek_main require statements for acts_as_asset and acts_as_isa