allow specification of the layer comp
add comp index support and update usage comments
lint cleanup
move layer comp settings out of layerSpec

so it is easier to target a given layer and layer comp w/o building a
layerSpec object
fix bug to allow use with layerId

jeeeezzzzzz (oops)
Add documentation of compId and compIndex settings to Generator.prototype.getPixmap; fix broken JSDoc on layerSpec.hidden
Merge pull request #235 from adobe-photoshop/jhatwich/layerComps

Jhatwich/layer comps