Use PUT for create!

This is as per the etcd api docs.

Using POST with {:dir true} was creating an extra node under the
directory.
Merge pull request #3 from hugoduncan/feature/fix-create

Use PUT for create!
Allow specification of a root key on get*

This allows access to keys such as /admin/machines which are not under
the /keys path by specify the :root-key option, e.g.
(get* connection "/machines" {:root-key "admin"}).

Note that this changes the signature of key-url, so adds an overload
to preserve the previous signature of the function.
Add url function

The key-url function forwards to the new general url function.
Remove :root-key from query-params