Merge pull request #309 from tomwayson/issue-295

added support for applying a rendering rule to an image layer when exporting an image
added support for setting mosaic rule on export image

added test to ImageMapLayerSpec.js
implemented getter/setter functions and updated `_buildExportParams` in ImageMapLayer.js
updated api reference page
added example page using lock rasters to show only one raster in a mosaic