Call get_last_seen with a specific key

Issue **[#5](https://github.com/BBC-News/alephant-sequencer/issues/5)** by @revett.

#### Problem

You have to create a new instance of a **Sequencer** each time you wish to use a different key (`ident`/`id`).

#### Solution

Allow `#get_last_seen` to take a key which overrides the `id` specified when the Sequencer was initialised, or if no key is given fall back to the default `id`.

```ruby
sequencer.get_last_seen
```

```ruby
sequencer.get_last_seen 'f284fu284jf'
```

![image.gif](http://i.imgur.com/ugbADiF.gif)
I'd hope we're all using `set noswapfile`... we'll be `.gitignore`ing `*.sw[a-p]` next! :sweat_smile: