Add getEmailSid

Other places sometimes need to re-use the email sid to send proof
of ownership of a email address to somewhere else.

src/interactive-auth.js

@@ -215,6 +215,14 @@ InteractiveAuth.prototype = {
         this._doRequest(auth, ignoreFailure);
     },
 
+    /**
+     * Gets the sid for the email validation session
+     * Specific to m.login.email.identity
+     */
+    getEmailSid: function() {
+        return this._emailSid;
+    },
+
     /**
      * Sets the sid for the email validation session
      * This must be set in order to successfully poll for completion