Added two make targets for Doxygen: "make doc" and "make doc_pages"

My Motivations are the following:
- the online documentation is pretty outdated and i want to give every developer an easy tool to generate her own documentation
- "make doc" is more convenient than "doxygen ../docs/Doxyfile" - you don't need to use yet another tool
- improve consistency:  mainpage.hpp.cmake is now configured into the cmake binary directory (see line 63 of CMakeLists.txt)

**Attention:**
This patch will negatively affect backwards compatibility because Doxyfile is renamed to Doxyfile.in and DoxyfilePages is renamed to DoxyfilePages.in. Moreover, the location of mainpage.hpp has changed.
As a result "doxygen ../docs/Doxyfile" no longer works.
I can also provide a backwards compatible patch if required.