Option to replace all items in database with imported items
Fix replace items fetch predicate
Handle deletion (and other) concurrency conflicts by explicitly locking main moc during sensitive operations
make sure main moc gets unlocked if bg has no changes
Move locking to only main moc
Fix minor bug with supplementary data option
Fix permanent ID issue with custom background import blocks
Refactor existing item deletion into shared method
Remove comment that doesn't make sense there
Merge pull request #22 from Raizlabs/ND-ReplaceItems

New options and bug fixes