Add a firstMatching function for CollectionType

CollectionType.swift

extension CollectionType {
    
    /// Returns the first element where `predicate` returns `true` for the
    /// corresponding value, or `nil` if such value is not found.
    ///
    /// - Complexity: O(`self.count`).
    func firstMatching(@noescape predicate: (Self.Generator.Element) -> Bool) -> Self.Generator.Element? {
        for (_, element) in self.enumerate() {
            if predicate(element) { return element }
        }
        return nil
    }
}