utils+apps: Fix a bug in ndn::RttEstimator

In the constructor of ndn::RttEstimator, the code tries to set
m_currentEstimatedRtt to m_initialEstimatedRtt using the ConstructSelf()
technique. However, it doesn't take effect at this point since
ndn::RttEstimator hasn't defined GetInstanceTypeId().

According to the manual
(http://www.nsnam.org/docs/manual/html/attributes.html#initialization-order),
"the object and all its derived classes must also implement a virtual
TypeId GetInstanceTypeId (void) const; method. Otherwise the
ObjectBase::ConstructSelf () will not be able to read the attributes".
GetInstanceTypeId() was defined in the original RttEstimator but was
somehow lost during the port to ndnSIM.

closes #40
diff --git a/utils/ndn-rtt-estimator.cc b/utils/ndn-rtt-estimator.cc
index 93193d2..b0d2b83 100644
--- a/utils/ndn-rtt-estimator.cc
+++ b/utils/ndn-rtt-estimator.cc
@@ -158,6 +158,12 @@
   NS_LOG_FUNCTION (this);
 }
 
+TypeId
+RttEstimator::GetInstanceTypeId (void) const
+{
+  return GetTypeId ();
+}
+
 void RttEstimator::SentSeq (SequenceNumber32 seq, uint32_t size)
 {
   NS_LOG_FUNCTION (this << seq << size);