Draft of attribute syntax API

I'm not totally sold on this. It means we have to check every permutation for a given input, e.g., for `key1.key2.key3`, we have to check for `node['key1.key2.key3']` and `node['key1.key2']['key3']` and `node['key1']['key2.key3']` and `node['key1']['key2']['key3']`. I prefer the simplicity of just using the array syntax to handle keys with dots. People can always alias an attribute via an ohai plugin or cookbook if they use it frequently enough.