Added return value control in Serial::SerialImpl::close () in unix.cc and win.cc

This Pull Request fixes issue #60.

Added return value control in Serial::SerialImpl::close () in unix.cc
If close() returns 0 (success) fd_ is set to -1, else if the return value is -1 (error) an exception is thrown.

Added return value control in Serial::SerialImpl::close () in win.cc
If CloseHandle() returns 0 (error) an exception is thrown, else (success) fd_ = INVALID_HANDLE_VALUE.