Update StructuredTable.php - default db column width from 50 increased to 255

This is a varchar column anyway. It does not hurt to increase the limit to 255. Otherwise Pimcore (mysql, infact) will truncate values added in the backend to 50 chars (we are storing urls in this column ..)
Merge pull request #61 from chluehr/patch-1

Update StructuredTable.php - default db column width from 50 to 255