Easy newsfeed and testing Headers at Adapter

-Fixed AsyncRSSParser to do the callbacks outside of the doInBackground,
thus not firing an exception
-Implemented the newsfeed the easy way: joining all posts together and
sorting them by date. Done so implementing Comparable at Post and
gathering the Posts at FeedList
-The non-selectable headers at the drawer are still not achievable as
proven by experiments FeedHeaderListAdapter and FeedPlainListAdapter,
that would substitute FeedListAdapter
-Created class PostContent to store contents of Posts, so as to relieve
the usual Feeds operation by removing the contents from regular Post
object. Still to polish.