Fixed issue where subscribing to a feed always returned an error

I am unable to subscribe to a feed...sort of.

```coffeescript
Superfeedr = require 'superfeedr'
feedClient = new Superfeedr 'user', 'pass'

feedClient.subscribe 'http://blog.superfeedr.com/atom.xml', (err, feed) ->
```

This does in fact subscribe the account to a feed, but returns an error anyway. It appears to have been introduced in commit 49261e05c3d73e604a8832bf70f547677e628567 at [Line 185](https://github.com/superfeedr/superfeedr-node/blob/master/lib/superfeedr.js#L185) where it attempts to obtain the child by the name of "subscribe" instead of "subscription"

If I am missing something please let me know.