Use an interface for tags. (#195)

* Use an interface for tags.

* rename TagSet to TagMap.

* add documentation to TagMap.

pubsub/query/empty.go

@@ -1,11 +1,13 @@
 package query
 
+import "github.com/tendermint/tmlibs/pubsub"
+
 // Empty query matches any set of tags.
 type Empty struct {
 }
 
 // Matches always returns true.
-func (Empty) Matches(tags map[string]interface{}) bool {
+func (Empty) Matches(tags pubsub.TagMap) bool {
 	return true
 }