Add Repositories.repository? to check if a repo exists.

I've used this method in a couple of places and I thought it could be useful for other users. My main user case is to check if a repo exists before creating it, something like this:

```ruby
client.create_repository('foo') unless client.repository?('calavera/foo')
```

Without `Repositories.repository?` I needed to capture the exception that `Repositories.repository` raises and the code was quite nasty.