NSPredicate filter

Syntax: `filter:"<nspredicate>"`
Examples:
`view filter:'tag=10'`
`view filter='text contains[cd] "something"'`

Includes unit tests.

Unfortunately, the diff is not well readable because I have renamed the original `SYPredicateFilter` to `SYSelectorFilter` and implemented a new `SYPredicateFilter` using `NSPredicate`. The implementation is pretty simple though.