Add inWaiting Property

Sources/SwiftSerial.swift

@@ -165,6 +165,11 @@ public enum BaudRate {
 }
 #endif
 
+#if os(OSX)
+// Darwin doesn't provide this
+private let FIONREAD: UInt = 0x541B
+#endif
+
 public enum DataBitsSize {
     case bits5
     case bits6
@@ -384,6 +389,12 @@ public class SerialPort {
             }
         }
     }
+    
+    public var inWaiting: Int? {
+        get{
+            return Int(ioctl(fileDescriptor!, FIONREAD))
+        }
+    }
 }
 
 // MARK: Receiving