Change CFileHandler::ifs from std::ifstream* to std::ifstream

This removes the unnecessary dynamic allocation, and also introduces
CFileHandler::Open() for reopening it. This fixes a potential bug
which might happen if CFileHandlers were able to simply be assigned
to one another, as that might double-delete CFileHandler::ifs.
Merge pull request #50 from Muon/depointerize-cfilehandler-ifs

Change CFileHandler::ifs from std::ifstream* to std::ifstream, see also
http://springrts.com/phpbb/viewtopic.php?f=12&t=29561