Fix list_display handling.

The django documentation for list_display lists four ways of specifying
elements: field names, callables, admin methods and model methods.

The current code in templatetags/mptt-admin.py takes only field names and
model methods into account: When the element is a callable, it throws a
ValueError in getattr(), when it is an admin method, it ignores it.

This commit fixes the issue by just using any field_name except
'action_checkbox' as mptt_indent_field.
Provide a section in the documentation that warns mptt users about the concurrency issues and about the fact that it's their responsibility for ensuring mutual exclusivity.

* this fixes issue #236
* also add a new link to an mptt tutorial (I find this more usefull than the existing ones as it also points out that CRUD operations need to be guarded by locking statements)
Merge pull request #242 from kux/issue_236

Documentation fix for issue #236
Merge pull request #229 from frederik-elwert/fix_228

Fix list_display handling.
Merge branch 'master' into update-things

Conflicts:
	mptt/templatetags/mptt_admin.py