Proposed fix for #335

Moved define to the constructor.

Tested in CI 2.0.3.1.

Doesn't matter if I use:
```
$excel = new PHPExcel();
```
or try to extend:
```
class E_PHPExcel extends PHPExcel {}
```

I get the following error:
```
Fatal error: Class 'PHPExcel_Calculation' not found in (...)/PHPExcel.php on line 360
```

Autoloader is not triggered, and I have to 'include_once'
 I have to include PHPExcel.php for example. Even though the class is found, the other classes are not loaded using the autoloader if i don't use 'require'/'include'_once PHPExcel.php explicitely

When I use the proposed solution, the classes are loaded using autoloader, without 'require' and excel is being generated

This seems to be working because:
http://stackoverflow.com/questions/2755500/php-require-class-call-from-inside-method