added IssueManager

- IssueManager performs validation on the current graph and also
   stores the issues and notifies listeners when the issues changed

modules/validations/issueManager.js

@@ -0,0 +1,22 @@
+import * as d3 from 'd3';
+import { utilRebind } from '../util/rebind';
+
+export function IssueManager(context) {
+    var dispatch = d3.dispatch('reload'),
+        self = {},
+        issues = [];
+
+    self.getIssues = function() {
+        self.validate();
+        return issues;
+    };
+
+    self.validate = function() {
+        var changes = context.history().changes();
+        issues = context.history().validate(changes);
+        dispatch.call('reload', self, issues);
+        return issues;
+    };
+
+    return utilRebind(self, dispatch, 'on');
+}