More options for gem users to pass in for initialization of the Savon cl...

...ient

@florrain can you :+1: ?
Purpose:  Looking to give gem users more flexibility for how they wish to interact with Responsys via Savon.

available_settings hash is pulled directly from [Savon globals](http://savonrb.com/version2/globals.html)

There's a quick check to see if the setting passed into the following initializer:
```ruby
Responsys.configure do |config|
  config.settings = {
    username: CREDENTIALS["username"],
    password: CREDENTIALS["password"],
    wsdl: CREDENTIALS["wsdl"],
    element_form_default: :qualified,
    ssl_version: :TLSv1
  }
```
is one of the available settings. If so, the Client Singleton is initialized with that key/value pair.
I think you're trying to use the `.select` method available on a Hash. What you could do is:
- make the available settings array a constant on top of the class.
- rename `client_settings` to `filtered_settings`. 
- the function would just return `settings.select {|k,v| k.to_s != "username" && k.to_s != "password" && available_settings.include?(k.to_s) }`