add CreatePlanet and put planet in solar system

models/planet.go

@@ -1,8 +1,10 @@
 package models
 
+import "gopkg.in/mgo.v2/bson"
+
 type Planet struct {
-	Id      string
-	Size    int64 // fields
+	Id      bson.ObjectId `json:"id", bson:"_id, omitempty"`
+	Size    int64         // fields/slots
 	Name    string
-	OwnerId string
+	OwnerId bson.ObjectId
 }