adding in new field types, should resolve Airtable#272 + Airtable#343

src/barcode.ts

@@ -0,0 +1,4 @@
+export interface Barcode {
+    type?: string;
+    text?: string;
+}

src/button.ts

@@ -0,0 +1,4 @@
+export interface Button {
+    label: string;
+    url: string;
+}

src/field_set.ts

@@ -1,14 +1,20 @@
 import {Collaborator} from './collaborator';
 import {Attachment} from './attachment';
+import {Barcode} from './barcode';
+import {Button} from './button';
 
 export interface FieldSet {
     [key: string]:
         | undefined
+        | null
         | string
         | number
         | boolean
         | Collaborator
+        | Barcode
+        | Button
         | ReadonlyArray<Collaborator>
         | ReadonlyArray<string>
+        | ReadonlyArray<number | string | boolean | any>
         | ReadonlyArray<Attachment>;
 }