gapi, etcd: Define and implement a string sharing API for the World

This adds a new set of methods to the World API (for sharing data
throughout the cluster) and adds an etcd backed implementation.

gapi/gapi.go

@@ -30,6 +30,11 @@ type World interface { // TODO: is there a better name for this interface?
 	ResExport([]resources.Res) error
 	// FIXME: should this method take a "filter" data struct instead of many args?
 	ResCollect(hostnameFilter, kindFilter []string) ([]resources.Res, error)
+
+	StrWatch(namespace string) chan error
+	StrGet(namespace string) (map[string]string, error)
+	StrSet(namespace, value string) error
+	StrDel(namespace string) error
 }
 
 // Data is the set of input values passed into the GAPI structs via Init.