Remove onNavigation and onBeforeNavigation from getDefaultProps

Fixes #22.

Author: andreypopp

lib/RouterMixin.js

@@ -16,13 +16,6 @@ var RouterMixin = {
     onNavigation: React.PropTypes.func
   },
 
-  getDefaultProps: function() {
-    return {
-      onBeforeNavigation: emptyFunction,
-      onNavigation: emptyFunction
-    };
-  },
-
   childContextTypes: {
     router: React.PropTypes.component
   },
@@ -156,13 +149,17 @@ var RouterMixin = {
    * @param {Callback} cb
    */
   setPath: function(path, navigation, cb) {
-    this.props.onBeforeNavigation(path, navigation);
+    if (this.props.onBeforeNavigation) {
+      this.props.onBeforeNavigation(path, navigation);
+    }
     this.replaceState({
       match: matchRoutes(this.getRoutes(this.props), path),
       prefix: this.state.prefix,
       navigation: navigation
     }, function() {
-      this.props.onNavigation();
+      if (this.props.onNavigation) {
+        this.props.onNavigation();
+      }
       cb();
     }.bind(this));
   },