Twitter feed component to display twitter feeds

Checkin for issue #131 

Added  twitter-feed component files and corresponding Java classes(TwitterFeedScheduler, TwitterFeedService, etc), the OSGi configuration is stored in config.twitter-feed folder. The TwitterFeedScheduler runs every 15 mins to pull the twitter feeds using twitter4j. By default the TwitterFeedScheduler is disabled cause of ConfigurationPolicy.REQUIRE.

To use/test, add the twitter-feed component from ACS Commons group to your page. From component dialog enter the twitter username. Enable the TwitterFeedScheduler by renaming the /apps/acs-commons/config.twitter folder to /apps/acs-commons/config.author. It will fetch twitter feed every 15mins and store on the resource under content tree.