Lowest common ancestor

[robotdana]

For finding the lowest common ancestor of a relation

[seuros]

@robotdana Thank you for your contribution. Can you give an use case for this feature ?
Also i see that you are unscoping the default scope. It will be problimatic for people that have default scopes that deleted records

[robotdana]

Our use case is finding the root to build navigation for a subset of records

lowest_common_ancestor is the most common treeish name for it

The default scope is easy to reapply, i've added a commit fixing it

But,default_scoped is an undocumented rails method, and this gem already has a warning about using default_scope (the only place the whole gem mentions default_scope)

### Does this work well with ```#default_scope```?

**No.** Please see [issue 86](https://github.com/ClosureTree/closure_tree/issues/86) for details.

So i initially thought the simpler version better

A version without using unscoped is over here with a different signature: master...robotdana:lowest_common_ancestor_non_chain

[robotdana]

default scoped is an undocumented rails method, and it looks like it works differently in different versions, closing because #328 is a safer version of this