Tweak the doxygen configuration (Doxyfile).
Add custom assert mechanism, update some unit tests.
Fix a signed/unsigned warning in Input.cpp.
Add Makefile.simple and Session.vim.

* Change the compiler flags in the Makefile.simple to `-Wall -ggdb3 -O0`
  (show all warnings, output debug symbols for GDB and no
  optimizations).
* Add Test.h and Test.cpp to Session.vim.
Merge branch 'develop' of github.com:nnen/cppapp into develop