Use context passed in as parameter instead of +MR_contextForCurrentThread

Previously, this method was using the context supplied as a parameter
to create the fetch request, but then used `[NSManagedObjectContext
MR_contextForCurrentThread]` to create the
`NSFetchedResultsController`.
It will now correctly use the provided context for both the fetch
request and the `NSFetchedResultsController`.
Added a 3rd condition for the migration error case
Update podspec to 2.3.0-beta 5
Update logging documentation for 2.3.0
Merge pull request #871 from Alydyn/develop

Use passed context parameter
Merge pull request #881 from birthdayalarm/develop

Check for NSMigrationError alongside NSPersistentStoreIncompatibleVersionHashError and NSMigrationMissingSourceModelError
Make parameter names consistent
Update documentation around Getting Started and iCloud
Add a few missing calls to super in the iOS sample project
Lots of updates to the documentation from the wiki (and some new content, too)