Poller element in http outbound adapter and gateway

This is just a proposal for a change. I searched around but did not see a JIRA issue for this, but felt that this is a good change just to ensure consistent behavior.

The proposal is to add a poller element to both http gateway and outbound adapter in their schema. Currently poller is supported at the level of http gateway and outbound adapter bean definition but not by their schema. This change attempts to fix the schema.

* Added in schema element for base poller, in both outbound adapter and outbound gateway
* Added in tests to ensure that a PollingConsumer is the instance type

Please review, if it looks okay I can open a jira and clean it much more based on any feedback.