modified to fix the iOS 7.0 compatibility issue

Since the isConnected method has been deprecated in iOS 7.0, replaced
with the state property

https://developer.apple.com/library/ios/documentation/CoreBluetooth/Refe
rence/CBPeripheral_Class/DeprecationAppendix/AppendixADeprecatedAPI.html
Merge pull request #22 from kotobuki/fixed-the-iOS-7.0-compatibility-issue

Fixed the iOS 7.0 compatibility issue