Moving apps back to apps/ folder.  Finally worked out the problem with headers

WAF building system copies all headers to one directory. As a result,
all #include directives in all NS3 modules should be either in form
"ns3/header-file.h" or just "header-file.h", disregarding the actual
directory structure.
diff --git a/wscript b/wscript
index 153e3a2..1670c53 100644
--- a/wscript
+++ b/wscript
@@ -16,8 +16,8 @@
         'model/interest-header.cc',
         'model/content-object-header.cc',
 
-        'model/stupid-interest-generator.cc',
-        'model/stupid-interest-sink.cc',
+        'apps/stupid-interest-generator.cc',
+        'apps/stupid-interest-sink.cc',
         ]
 
     module_test = bld.create_ns3_module_test_library('NDNabstraction')
@@ -44,8 +44,8 @@
         # 'helper/ndnabstraction-helper.h',
         'helper/ndn_stupidinterestgenerator_helper.h',
 
-        'model/stupid-interest-generator.h',
-        'model/stupid-interest-sink.h'
+        'apps/stupid-interest-generator.h',
+        'apps/stupid-interest-sink.h'
         ]