split query proxy methods into their own spec, add delete_all for qp

Addresses #557.

`delete_all` removes all matching relationships but does not delete nodes. Its behavior is the same as ActiveRecord's method of the same name. Cypher makes this easy, it's really just a convenience method for something like `student.lessons(:l, :r).query.delete(:r)`.

Specs look like they're failing after pushing but they aren't locally. I have to run, I'll have to look later to figure out why.