Tightened up imageloader dependencies - doesn't need all of node

Fixed imageloader dependencies in YUI.add and loader meta-data. Also found a dependency bug in base-base as a result (assumed Y.Plugin always exists, which it doesn't until plugin or pluginhost are pulled in), which is fixed also.

Optimized imageloader requires: node pulls in base and event. oop pulled in by event-custom

ImageLoader 3.0 conversion