Fix #29 C++ Style Includes

- c std libs should be included <cLIBNAME> in C++
  http://www.cplusplus.com/reference/clibrary/
- gcc 4.8+ failed because pngwriter uses cstring and
  not c++ string which are different libraries
  (but probably pulled some indirect includes before)
Link libm explicitly, too
Remove Docs to OLD_CPP
Merge pull request #30 from ax3l/fix-includes

Fix #29 C++ Style Includes