Improve syncing based on recent XMLAPI changes

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Save the datetime the collection sync runs (in the shared preferences)
Add a modifiedsince=YYYY-M-D parameter to the collection URL based on the last 
time the sync ran. (leave off if never run)
In the collection feed, parse and save the items.item.status.lastmodified 
field. (not necessary, but can be useful for refreshing)

Don't delete items from the collection after this sync, because there will be 
un-updated games that should be left in the DB. Need to think about how to 
remove games from our DB that have been removed from BGG. Perhaps, we can query 
the entire collection, only looking for the ID, then update that record, and 
delete them as before. Use XMLAPI2's brief mode: 
http://boardgamegeek.com/xmlapi2/collection?username=jester&brief=1

Original issue reported on code.google.com by chris.co...@gmail.com on 17 Jun 2011 at 5:19

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Original comment by chris.co...@gmail.com on 30 Jun 2011 at 7:59

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Original comment by chris.co...@gmail.com on 9 Jan 2012 at 4:16

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