Support new etcd 0.2 response data stucture

Etcd 0.2 nest data inside the node key, also directory entries can have multiple nodes with recursive content, this PR address that as well rearrange the specs. It introduces one breaking change i.e. removal of `index` method from the keys/nodes. since etcd itself has changed the data format, i think its fine to introduce this breaking change. instead now we'll expose `created_index` and `modified_index` via the node object. If the one of these index values resembles the old `index` method in its entirety, we can reintroduce the index method before cutting a gem that delgates to nodes `create_index` or `modified_index`, but this should provide a good base for supporting the new lock, leader module.