Share Dialog does not require Login or any extended permissions

Currently user have to have an active session with Facebook plugin to use **"Share Dialog"**, but Facebook documentation suggest *". This does not require Facebook Login or any extended permissions, so it is the easiest way to enable sharing on web."*

This Change just request an active session when developer execute a showDialog different than a share dialog
Share Dialog does not require Login or any extended permissions

If is a Share dialog but FB app is not installed the WebDialog Builder fails. In Android all WebDialogs require a not null Session object.

Documentation for Android SDK FeedDialog:
https://developers.facebook.com/docs/reference/android/3.0/class/WebDialog.FeedDialogBuilder/

**session** The Session representing an authenticating user to use for showing the dialog; must not be null, and must be opened.
Merge pull request #851 from jcvalerio/patch-2

Share Dialog does not require Login or any extended permissions