Add support for portlets

[tehapo]

Add support for using vaadin4spring in portlet applications.

[peholmst]

This will be done at some point, but I don't know when. Right now the emphasis is on getting everything working properly for servlet applications. Some actions have been taken already to make the migration to a portlet environment a little easier.

[Catchwa]

This was fixed in #207 but it would be good if a sample could also be added to the samples directory as I'm currently struggling to port my existing Vaadin 6 + Liferay 6 application to Vaadin 7 + Liferay 7