Updated Gedmo tree column type
Merge pull request #13 from norzechowicz/tree

Updated Gedmo tree column type
Changed defualt translation_domain option value to messages
Added possibility to translate column header labels
Fixed typo
Merge pull request #14 from norzechowicz/dev

Added possibility to translate export columns headers.
Expanded action column default block to handle url_attr and content
Merge pull request #15 from norzechowicz/dev

Expanded action column default block to handle url_attr and content