Rewrite module function declaration & export to match template

Follows template in TS docs now:
https://www.typescriptlang.org/docs/handbook/declaration-files/templates/module-function-d-ts.html

Also makes it possible to reference interfaces from the NodeVault
namespace outside of this module.

types/node-vault/index.d.ts

@@ -122,7 +122,5 @@ declare namespace NodeVault {
     }
 }
 
-type GetClient = (options?: NodeVault.VaultOptions) => NodeVault.client;
-
-declare const getClient: GetClient;
-export = getClient;
+declare function NodeVault(options?: NodeVault.VaultOptions): NodeVault.client;
+export = NodeVault;

types/node-vault/node-vault-tests.ts

@@ -1,7 +1,7 @@
 import nv = require("node-vault");
 
 // Test code came from the sample code in README of the module.
-const options = {
+const options: nv.VaultOptions = {
   apiVersion: 'v1', // default
   endpoint: 'http://127.0.0.1:8200', // default
   token: '1234', // optional client token; can be fetched after valid initialization of the server