feat(secure-storage): Add Secure Storage Wrapper (#347)

* Add SecureStorage plugin.

* Add SecureStorage plugin.

* Added create() and removed init() to provide more consistency to ionic-native plugins.
Edited comments to reflect changes.

* Removed init and comment edit.

src/index.ts

@@ -62,6 +62,7 @@ import {Printer} from './plugins/printer';
 import {Push} from './plugins/push';
 import {SafariViewController} from './plugins/safari-view-controller';
 import {Screenshot} from './plugins/screenshot';
+import {SecureStorage} from './plugins/securestorage';
 import {Sim} from './plugins/sim';
 import {SMS} from './plugins/sms';
 import {SocialSharing} from './plugins/socialsharing';
@@ -139,6 +140,7 @@ export {
   OneSignal,
   PinDialog,
   Screenshot,
+  SecureStorage,
   SocialSharing,
   Sim,
   Splashscreen,
@@ -211,6 +213,7 @@ window['IonicNative'] = {
   PinDialog: PinDialog,
   SafariViewController: SafariViewController,
   Screenshot: Screenshot,
+  SecureStorage: SecureStorage,
   Sim: Sim,
   SMS: SMS,
   SocialSharing: SocialSharing,