Added @Produces annotations (fixes an issue in Wildfly 8b1)

This works around the following exceptions:

21:37:17,547 WARN  [org.jboss.resteasy.core.ExceptionHandler] (default task-17) Failed executing POST /applications/2086d764-a9c9-4bb1-ac98-94265c65ba3c/android: org.jboss.resteasy.core.NoMessageBodyWriterFoundFailure: Could not find MessageBodyWriter for response object of type: org.jboss.aerogear.unifiedpush.model.AndroidVariant of media type: application/octet-stream
	at org.jboss.resteasy.core.ServerResponseWriter.writeNomapResponse(ServerResponseWriter.java:67) [resteasy-jaxrs-3.0.4.Final.jar:]
	at org.jboss.resteasy.core.SynchronousDispatcher.writeResponse(SynchronousDispatcher.java:411) [resteasy-jaxrs-3.0.4.Final.jar:]
	at org.jboss.resteasy.core.SynchronousDispatcher.invoke(SynchronousDispatcher.java:376) [resteasy-jaxrs-3.0.4.Final.jar:]
	at org.jboss.resteasy.core.SynchronousDispatcher.invoke(SynchronousDispatcher.java:179) [resteasy-jaxrs-3.0.4.Final.jar:]
	at org.jboss.resteasy.plugins.server.servlet.ServletContainerDispatcher.service(ServletContainerDispatcher.java:220) [resteasy-jaxrs-3.0.4.Final.jar:]
	at org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:56) [resteasy-jaxrs-3.0.4.Final.jar:]
	at org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:51) [resteasy-jaxrs-3.0.4.Final.jar:]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) [jboss-servlet-api_3.1_spec-1.0.0.Beta1.jar:1.0.0.Beta1]
	at io.undertow.servlet.handlers.ServletHandler.handleRequest(ServletHandler.java:87) [undertow-servlet-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.servlet.handlers.security.ServletSecurityRoleHandler.handleRequest(ServletSecurityRoleHandler.java:59) [undertow-servlet-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.servlet.handlers.ServletDispatchingHandler.handleRequest(ServletDispatchingHandler.java:36) [undertow-servlet-1.0.0.Beta17.jar:1.0.0.Beta17]
	at org.wildfly.extension.undertow.security.SecurityContextAssociationHandler.handleRequest(SecurityContextAssociationHandler.java:81)
	at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:25) [undertow-core-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.servlet.handlers.security.SSLInformationAssociationHandler.handleRequest(SSLInformationAssociationHandler.java:113) [undertow-servlet-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.security.handlers.AuthenticationCallHandler.handleRequest(AuthenticationCallHandler.java:52) [undertow-core-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.security.handlers.AbstractConfidentialityHandler.handleRequest(AbstractConfidentialityHandler.java:45) [undertow-core-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.servlet.handlers.security.CachedAuthenticatedSessionHandler.handleRequest(CachedAuthenticatedSessionHandler.java:65) [undertow-servlet-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.security.handlers.SecurityInitialHandler.handleRequest(SecurityInitialHandler.java:70) [undertow-core-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:25) [undertow-core-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:25) [undertow-core-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.servlet.handlers.ServletInitialHandler.handleFirstRequest(ServletInitialHandler.java:218) [undertow-servlet-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.servlet.handlers.ServletInitialHandler.dispatchRequest(ServletInitialHandler.java:205) [undertow-servlet-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.servlet.handlers.ServletInitialHandler.access$000(ServletInitialHandler.java:69) [undertow-servlet-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.servlet.handlers.ServletInitialHandler$1.handleRequest(ServletInitialHandler.java:134) [undertow-servlet-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.server.HttpHandlers.executeRootHandler(HttpHandlers.java:36) [undertow-core-1.0.0.Beta17.jar:1.0.0.Beta17]
	at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:619) [undertow-core-1.0.0.Beta17.jar:1.0.0.Beta17]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [rt.jar:1.7.0_45]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [rt.jar:1.7.0_45]
	at java.lang.Thread.run(Thread.java:744) [rt.jar:1.7.0_45]