[FCModel numberOfInstancesWhere:arguments:] method

Swift didn't want to play nice with varargs in `-numberOfInstancesWhere:` (does it work for anyone else?), so I set to create an array version of it.

A common base for both old and new methods, `(NSUInteger)_numberOfInstancesWhere:(NSString *)queryAfterWHERE withArgumentsInArray:(NSArray *)array orVAList:(va_list)args`, was thus produced by analogy with `(id)_instancesWhere:(NSString *)query andArgs:(va_list)args orArgsArray:(NSArray *)argsArray orResultSet:(FMResultSet *)existingResultSet onlyFirst:(BOOL)onlyFirst keyed:(BOOL)keyed`.

*Disclaimer: I tested both methods, but `va_list` is new for me, so a careful second look might be in order.*