Implemented watching using filter package

* Added filters / watches
* Removed event dep

whisper/filter.go

@@ -0,0 +1,10 @@
+package whisper
+
+import "crypto/ecdsa"
+
+type Filter struct {
+	To     *ecdsa.PrivateKey
+	From   *ecdsa.PublicKey
+	Topics [][]byte
+	Fn     func(*Message)
+}