Add 'public_key' property to Recurly.js

Additional property `public_key` for `Recurly.js` class. 
It enables you to store `Public Key` when you configure `recurly-client-ruby` gem and then use it for recurly.js configuration.

This property is not supposed to be used directly in `recurly-client-ruby` gem, we just store the value to make it easy accessible later for [Recurly.js](https://js.recurly.com) configuration.

Let me show you how it can be used:
- store public key in your `config/initializers/recurly.rb` file 

``` ruby
Recurly.subdomain      = 'domain'
Recurly.api_key        = '111'
Recurly.js.public_key  = '222'
```
        
- easy use it `assets/javascripts/recurly_service.js.erb` of r[configure](https://docs.recurly.com/js/#configure) recurly.js

``` ruby
recurly.configure({ publicKey: '<%= Recurly.js.public_key %>'});
```

I found it really useful :+1: