add `get_real_fields()` function to check whether column is real or fake field.

add variable `$real_fields` and `get_real_fields()` function to check whether column is real or fake field.

For normal situation, with callback column, which is a fake field, it will be error **"unknwon column"**  when we try to search with this 'field'.  

I have modified `customers_management` function by adding CreditCategory as callback_column.  And when I search by this column (CreditCategory), it throws Error Number 1054.

![unknown column](https://f.cloud.github.com/assets/2386359/1670450/28a2e3fa-5c97-11e3-8640-ee83dccd5f50.PNG)

I have modified **list_template.php** in flexigrid views to filter that only real field will be available to be searched.

Hi @blakdronzer, does this solve your issue #229 ?