fix week_index() 64-bit warning
added date_range::intersect()
fix mac build error
more iphone work

 - split ActFileManager out of ActDatabaseManager, deals with all the
   dropbox stuff. ActDatabaseManager is now instantiated by
   ActActivitiesViewController as it needs it.

 - added untested code to ActFileManager to handle uploads

 - added untested code to ActDatabaseManager to synchronize modified
   activities back to the remote file store

 - more random UI tweaks