Add a default ctor; due to 0 being magic, PID pid(0) is ambiguous.

git-svn-id: https://arducopter.googlecode.com/svn/trunk@959 f9c3cf11-9bcb-44bc-f272-b75c42450872

libraries/PID/PID.h

@@ -12,6 +12,14 @@
 /// @brief	Object managing one PID control
 class PID {
 public:
+	/// Constructor
+	///
+	/// A PID constructed in this fashion does not support save/restore.
+	///
+	PID() :
+		_address(0)
+	{}
+
 	/// Constructor
 	///
 	/// @param address	EEPROM base address at which PID parameters