Fixed bugs & warnings on XCode 4.5.2

Fixed a number of issues which prevent this library from compiling on XCode 4.5.2.  The current branch doesn't compile because of simultaneous use of assign and __weak directives on properties in GCJsonStreamWriter and GCJsonStreamParser, and an invalid block signature in GCDateFilteredUploadPicker.  

Also fixed a number of type warnings and format strings.