revise option notes

README.md

@@ -194,10 +194,10 @@ var oauth = OAuth(/* options */);
 }
 ```
 
-* ``signature_method``: ``String`` default ``HMAC-SHA1``
+* ``signature_method``: ``String`` default ``'HMAC-SHA1'``
 * ``nonce_length``: ``Int`` default ``32``
-* ``version``: ``String`` default ``1.0``
-* ``parameter_seperator``: ``String`` for header only, default ``, ``. Note that there is a space after ','
+* ``version``: ``String`` default ``'1.0'``
+* ``parameter_seperator``: ``String`` for header only, default ``', '``. Note that there is a space after ``,``
 * ``last_ampersand``: ``Bool`` default ``true``. For some services if there is no Token Secret then no need ``&`` at the end. Check [oauth doc](http://oauth.net/core/1.0a/#anchor22) for more information
 
 > oauth_signature is set to the concatenated encoded values of the Consumer Secret and Token Secret, separated by a '&' character (ASCII code 38), even if either secret is empty