Post contents now shown, and stored in cache

-Posts now don't have their contents in the main object. There is an
auxiliary object, PostContent, which holds and shows Post contents,
which are saved to and retrieved from cache memory. Unfortunately to
achieve this, Context must now be circulated all around...
-Post content fragment is now working and showing text contents
-All static final String TAG members of each class now dynamically get
their class' names (only debugging purpose)
FeedList to instance, ProgressDialog onsourceadd

-Added a ProgressDialog when adding a new source
-Corrected FeedListAdapter.indexOf, which was returning wrong value (the
value of the original list, without minding the special items and
headers)
-Now the main FeedList is saved onSaveInstanceState and restored
onRestoreInstanceState so that drawer is not resetted after an
orientation change
-Re-arranged startup methods to allow the app to differentiate whether
starting fresh or restoring saved instance.
-New base for load-values infrastructure: only username will be saved,
and based on that, feedlist will be retrieved from internal memory
-Exceptions at download/parse time are now passed up to MainActivity
(this might need to be rolled back...)