detail: move common.hpp to 'detail'

common.hpp and config.hpp are implementation details that should be
installed to target system.

common-pch.hpp is moved to 'impl'.

This commit also adds README.md to 'detail' and 'impl'
directories to explain their semantics.

refs #4782

Change-Id: I4f3e607338b3bdccbaddaed735f6326adcc2fda0
diff --git a/tests/wscript b/tests/wscript
index bc540ee..8e269f0 100644
--- a/tests/wscript
+++ b/tests/wscript
@@ -7,7 +7,7 @@
     bld.objects(target='tests-common',
                 features='pch',
                 source=bld.path.ant_glob('*.cpp'),
-                headers=['../ndn-cxx/common-pch.hpp', 'boost-test.hpp'],
+                headers=['../ndn-cxx/impl/common-pch.hpp', 'boost-test.hpp'],
                 use='ndn-cxx BOOST')
 
     bld.recurse('integrated')