Clarify that the default char set returned is base64 encoded

Also, link to a neutral URL for the MIT license instead of the node-ses repo.

README.md

@@ -8,7 +8,9 @@ Node.js module that generates a cryptographically secure random string with a gi
 var srs = require('secure-random-string');
 ```
 
-### Default behavior: Generate a random string 32 characters long.
+### Default behavior: Generate a random Base64 encoded string 32 characters long.
+
+This may include alphanumeric characters as well as the following characters: +, /, =.
 
 ```javascript
 // Sync
@@ -59,9 +61,9 @@ the async API returns the error to the callback.
 
 ## Contributors
 
- [Mark Stosberg](https://github.com/markstos)
+ [Mark Stosberg](https://github.com/markstos)  
  [Sandro Gomez](https://github.com/mrsangrin)
 
 ## License
 
-[MIT](https://github.com/aheckmann/node-ses/blob/master/LICENSE)
+[MIT](https://opensource.org/licenses/MIT)

tests.js

@@ -56,6 +56,14 @@ srs({alphanumeric: true}, function(err, sr) {;
 		true
 	);
 });
+
+srs({alphanumeric: true, length: 40}, function(err, sr) {;
+	test('Must contain alphanumeric only and be 40 chars long',
+		sr.match(/^[a-zA-Z0-9_]*$/g)[0] === sr && sr.length === 40,
+		true
+	);
+});
+
 // sync tests
 test('generate a random string 32 chars long (sync)', srs().length, 32);
 test('generate a random string 1 chars long (sync)', srs({length: 1}).length, 1);