Add ability to create a client with a token

Allows creating a client with a token directly.

```ruby
Gocoin::Client.new(token: 'saved_token')
```

Rather than:

```ruby
c = Gocoin::Client.new
c.token = 'saved_token'
```

Sorry about the unrelated whitespace fixes, my editor automatically removes trailing whitespace and adds a newline at EOF. Since its such a small PR and removing the whitespace is good, I figured you probably wouldn't mind.