Proposal for Support CORS (Cross-Origin Resource Sharing) #216

Add a Filter to handle multiple resources and a Helper to add CORS headers manually.
Typically, different resources are going to support different methods (e.g. resource A might support GET/PUT, while resource B might support GET/POST).  Therefore, configuring the filter with a single standard set of allowed methods doesn't seem useful to me.  Instead, I suggest that you have a default behavior of allowing all methods implemented on a given resource (those annotated with @Get, @Put, etc), as well as a way for developers to override that on a per-request, per-resource basis.  Therefore I suggest you use response.getAllowedMethods().  By default, this will be populated to reflect the annotated methods on the ServerResource, when ServerResource.options() calls updateAllowedMethods(), but a given ServerResource can modifiy that set of methods if desired in its @Options-annotated method.