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
Beginning a new TestContextManager to assist better unit testing

No incremental upgrade changes yet
Readded CoreDataConcurrencyTest to the test target
Reader: Prevents flickering after loading more posts
Notifications: Prevents SectionFooter from rendering
NoteTableHeaderView: Updates Style
NoteTableHeaderView: Fixes incorrect height assignment
NoteTableHeaderView: Implements custom separator
NoteTableHeaderView: Updates Style. Part II
NoteTableHeaderView: Updates Style. Part III
Make saveContext a class method rather than an instance method
Merge pull request #2569 from wordpress-mobile/issue/fixes-reader-load-more-flicker-on-ipad

Reader: Prevents flickering after loading more posts
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
Switched back to instance save methods on ContextManager

Also removing CoreDataTestHelper and it's horrible swizzled methods.
Fixed problem with shared instance being nil unexpectedly

Disabled two unit tests because they misbehave with Notifications being fired off
Fixed the precompiled header for WPiOS.