fw+doc: Renaming and extending limit extenstion

SimpleLimits now called PerOutFaceLimits.  Also, there is now
PerFibLimits that is inherited from PerOutFaceLimits and implements
additional check.

Also, this commit includes a couple of tricks to improve documentation
of templated classes (PerOutFaceLimits and PerFibLimits). Not ideal, but
much better than before.
diff --git a/model/pit/ndn-pit-entry-incoming-face.h b/model/pit/ndn-pit-entry-incoming-face.h
index cd75643..fb3987e 100644
--- a/model/pit/ndn-pit-entry-incoming-face.h
+++ b/model/pit/ndn-pit-entry-incoming-face.h
@@ -52,7 +52,7 @@
    */
   IncomingFace ();
   /**
-   * @brie Copy operator
+   * @brief Copy operator
    */
   IncomingFace &
   operator = (const IncomingFace &other);
diff --git a/model/pit/ndn-pit-entry-outgoing-face.h b/model/pit/ndn-pit-entry-outgoing-face.h
index 63c6378..20dfdaf 100644
--- a/model/pit/ndn-pit-entry-outgoing-face.h
+++ b/model/pit/ndn-pit-entry-outgoing-face.h
@@ -56,7 +56,7 @@
   OutgoingFace ();
 
   /**
-   * @brie Copy operator
+   * @brief Copy operator
    */
   OutgoingFace &
   operator = (const OutgoingFace &other);