Add step-by-step guide for parsing signed_request

Added detailed implementation guide for parsing signed_request.
These guides are directly copied from official document.
Add response_type option to auth_uri()
Add some comments regarding OAuth 2.0
misc refactoring
Add comments about appsecret_proof

Now that offcial document about appsecret_proof is provided,
detailed description is added.
Update link to Field Expansion document
Update document for redirect_uri
misc document change
Update introduction
Add exchange_token to get long-lived user token
Checking in changes prior to tagging of version 1.13.

Changelog diff is:

diff --git a/Changes b/Changes
index 774ea6f..993ec6d 100644
--- a/Changes
+++ b/Changes
@@ -2,6 +2,11 @@ Revision history for Perl extension Facebook::OpenGraph

 {{$NEXT}}

+1.13 2014-04-13T02:55:26Z
+    - Add exchange_token() to aqcuire long lived user token
+    - Update links to official document
+    - Add more comments for each implementation
+
 1.12 2014-02-27T16:40:08Z
     - Fixed test comments
     - Updated document URLs
Checking in changes prior to tagging of version 1.13.

Changelog diff is:

diff --git a/Changes b/Changes
index 993ec6d..883db8c 100644
--- a/Changes
+++ b/Changes
@@ -2,7 +2,7 @@ Revision history for Perl extension Facebook::OpenGraph

 {{$NEXT}}

-1.13 2014-04-13T02:55:26Z
+1.13 2014-04-13T03:03:03Z
     - Add exchange_token() to aqcuire long lived user token
     - Update links to official document
     - Add more comments for each implementation