Add distinct class names to imagemapster layers

[jamietre]

Creating complex effects can be accomplished by working directly with the layers ImageMapster creates. This currenlty involves targeting them by index position which is brittle and could easily break if (for example) layers were added by snapshot. Additionally it doesn't work with VML, which actually some things could work interchangeably.

Add class names to layers and unique IDs to snapshots. Use underscores to distinguish from end-user classes.

// .mapster_el and #mapster_wrap_n already exist as shown. The others will be added

<div id="mapster_wrap_1" class="mapster_wrap">
   <image id="original-image" class="mapster_source mapster_el">
   <canvas id="mapster_snapshot_1" class="mapster_snapshot mapster_el" />
   <canvas class="mapster_select" mapster_el />
   <canvas class="mapster_highlight" />
</div>