Flattr queue

These two patches implement a queue for pending flattrs whenever no internet connection is available. All pending items are flattred when the next thing is flattred and an internet connection is available.

I would consider this a first step towards implementing danieloeh/AntennaPod#87

The second patch creates a new flattr_queue table in the database and uses it to store the items to be flattred. This bumps the DATABASE_VERSION to 9.

Items in the table (CREATE TABLE FlattrQueue (id INTEGER PRIMARY KEY AUTOINCREMENT, feedItem INTEGER, feed INTEGER)) may either be a pending flattr to a feedItem (feedItem nonzero, feed is ignored), a feed (feeditem must be zero/NULL) or to AntennaPod itself (both feedItem and feed are zero).