Fix windows build error by including string header.

//------
Windows build error was:

  1>..\..\..\CarreraSlice\src\sfm_local_chanvese\src\interactive_kurvolver.cpp(140) : error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'std::string' (or there is no acceptable conversion)
  1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\ostream(653): could be 'std::basic_ostream<_Elem,_Traits> &std::operator <<<char,std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,const char *)' [found using argument-dependent lookup]
  1>        with
  1>        [
  1>            _Elem=char,
  1>            _Traits=std::char_traits<char>
  1>        ]
  [...]
//------

Also consistently use std::cout / std::endl, calling "using namespace std;"
is considered bad practice. See [1] for more details.

[1] http://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice?answertab=votes#tab-top