Add category service in Angular2.

src/operations/category.service.ts

@@ -0,0 +1,22 @@
+// vim: set tw=80 ts=2 sw=2 sts=2:
+
+import { Injectable } from '@angular/core';
+import { Observable } from 'rxjs/Rx';
+
+import { Restangular } from "ngx-restangular";
+
+import { Category } from './category';
+
+@Injectable()
+export class CategoryService {
+  constructor(
+    private restangular: Restangular
+  ) {}
+
+  query(id: number, minDate: Date = null, maxDate: Date = null): Observable<Category[]> {
+    return this.restangular.one('account', id).getList('category', {
+      begin: minDate,
+      end: maxDate
+    });
+  }
+}