Fixed compatibility with Nette 2.2 Callback
Fixed infinite recursion
Fixed support for primaryValue which == NULL [closes #30][closes #34]
Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/Datagrid.php
Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/Datagrid.php
compatibility with latte 2.2.2