Added return type on typescript defs for enc/dec

Author: shukob

dist/secure-ls.d.ts

@@ -35,7 +35,7 @@ declare class SecureLS {
 declare namespace SecureLS{
     interface Base64 {
         _keyStr: string;
-        encode(e: string);
-        decode(e: string);
+        encode(e: string): string;
+        decode(e: string): string;
     }
 }