add recursive attribute for delete

I always end up making my own recursive delete function for znodes. I decided I would build it in to the delete function directly here.

You can do:

z.create(:path=>"/test")
z.create(:path=>"/test/level1")
z.create(:path=>"/test/level1/level2")
z.delete(:path=>"/test", :recursive=>true) # => {:req_id=>12, :rc=>0}