Safer migration, boolean description fix.

[ADDED]     Safer store migration: don't delete the old store, allowing the client to downgrade.
[ADDED]     Log checkpoints and send to TestFlight too.
[FIXED]     Describe booleans as YES/NO, not 1/0.
Merge branch 'master' of github.com:Lyndir/MasterPassword
Work-around for a weird Core Data bug.
Fix sign comparison of enum.
Merge branch 'master' of github.com:Lyndir/MasterPassword
Mac: New sites.

[MOVED]     Creation of new elements moved to shared code.
[FIXED]     When switching user, unset active key.
[FIXED]     Synchronize content calculation to avoid race issues while typing.
[ADDED]     Ability to create new sites.
[FIXED]     Unset active element when hitting backspace or escape.