Added concept of default Blog for an Account.

Still need to conditionally set the value instead of always setting it.  Also, need to move the selection for Today into AccountService, most likely.
Autoset the Today Widget for default account when wp.com
Added WIDGETS_EXIST to AccountService
Changed to NSParameterAsserts
Switched to WIDGETS_EXIST macro
Merge pull request #2555 from wordpress-mobile/issue/2546-default-dotcom-today

Set default blog when syncing blogs for Today Widget