Added the donation classes

ref #1

src/models/Donation.ts

@@ -0,0 +1,32 @@
+import {
+  IsInt,
+  IsNotEmpty,
+  IsObject
+} from "class-validator";
+import { Donor } from './Donor';
+
+/**
+ * Defines the Donation base calss.
+ * A donation just associates a donor with a donation amount.
+ * The specifics of the amoun's determination has to be implemented in child classes.
+*/
+export abstract class Donation {
+  /**
+   * Autogenerated unique id (primary key).
+   */
+  @IsInt()
+  id: number;
+
+  /**
+   * The donations's donor.
+   */
+  @IsNotEmpty()
+  @IsObject()
+  donor: Donor;
+
+  /**
+   * The donation's amount in cents (or whatever your currency's smallest unit is.).
+   * The exact implementation may differ for each type of donation.
+   */
+  public abstract get amount(): number;
+}