Add ability to grab addresses by range as well as by net.

Since it is possible for address ranges to have their own conduit
settings, it is a good idea for us to let the nethelper code we inject
into the Chef node object be able to use it.
Add special-case logic to node to handle "admin".

It is rather useful to be able to refer to the admin node using an
easy-to-remember name.  "admin" is nice.

This lets API calls to use "admin" to refer to the admin node without
hardcoding a name or a node ID.
Fix typo in api_helper.rb