Merge pull request #6 from kenmyrs/master

Support for subqueries.
Made the actual saved identifier be stored in a separate ivar
Simplified where handling, made it accept predicates
made select query set the `savedIdentifier`
Merge branch 'master' of https://github.com/fjolnir/DatabaseKit

# By Ken Myers
# Via Fjölnir Ásgeirsson (1) and Ken Myers (1)
* 'master' of https://github.com/fjolnir/DatabaseKit:
  Support for subqueries.
Silenced some warnings
Cleaned up DBQuery
Fixed typo
added isInserted getter
Made where variadic, and take an NSPredicate string
Made outErr optional
Updated readme
Fixed subquery select
Made LIKE  predicate support * and ?
Filtering is now solely done through use of predicates
Made dbtable where: return a select query
enabled foreign keys by default
fixed delete query
Handled negation for IN operator
Made destroy inert if not inserted