A plugin for displaying OpenLayers images as a background

This plugin is a proof of concept, and so it is at a very first stage.

What it does: 
    * It downloads a Yahoo satellite background image of the current view port (well, sort of)

What it does NOT:
    * Everything else :-)
 
What needs to be done:
    * Fix positions
    * Fix refresh problems
    * Patch JOSM to restrict zoom to discrete steps (needed by Yahoo)
    * Get rid of ehcache. It's too bulky for our needs.
    * Add configuration options.
        * Allow the user to add any kind of OpenLayer layers
        * Add options for cache (size, expiration time...)
    * Find some clever way of distributing dependency libraries. A plugin of 2.1Mb seems too big.
