?Keys=XXX&separator="/" support

From the Consul API [docs](http://www.consul.io/docs/agent/http.html) 
```
It is possible to also only list keys without their values by using the "?keys" query parameter along with a GET request. This will return a list of the keys under the given prefix. The optional "?separator=" can be used to list only up to a given separator.

For example, listing "/web/" with a "/" seperator may return:

[
"/web/bar",
"/web/foo",
"/web/subdir/"
]
Using the key listing method may be suitable when you do not need the values or flags, or want to implement a key-space explorer.
```

This adds the optional named separator argument to the find method to support the API feature. In that case the return type changes to a list (which is weird).

I could implement this as a different method (keys?) if you'd prefer.