added support for image service rendering rule

added test to spec for getter/setter functions
implemented getter/setter functions
updated _buildExportParams() to pass renderingRule if option is set
updated api reference page to include new option and getter/setter functions
added example page applying a hillshade to a DEM.

resolves #295
Add Rendering Rules page to examples TOC
Merge pull request #309 from tomwayson/issue-295

added support for applying a rendering rule to an image layer when exporting an image