Added CheckboxTree and extended Tree to find node by name

- you can now search nodes in whole tree with find_node_by_name(),
this method returns subtree, so you can chain it:
find_node_by_name('root).find_node_by_name('some nested node')
- ChckboxTree is just a small extension of Tree class
Added product_features property into new/edit role forms
Merge pull request #26 from jprovaznik/checkbox1

checkbox tree