misc document update
Add $res->etag fixed #3
Add misc document regarding error handling
Merge branch 'feature/response/etag'
Checking in changes prior to tagging of version 1.20.

Changelog diff is:

diff --git a/Changes b/Changes
index 883db8c..a80eb49 100644
--- a/Changes
+++ b/Changes
@@ -2,6 +2,11 @@ Revision history for Perl extension Facebook::OpenGraph

 {{$NEXT}}

+1.20 2014-04-24T13:56:31Z
+    - Add js_cookie_name and get_user_token_by_cookie to work with JS SDK
+    - Add $res->etag and $res->header($field_name) to access response header
+    - Add more documentation
+
 1.13 2014-04-13T03:03:03Z
     - Add exchange_token() to aqcuire long lived user token
     - Update links to official document