Adds order methods, fixes SSL

## What

It's quite possible that connecting via SSL never worked. Adds many tests so `Connection` supports the various ssl_* configuration options.

Also adds method `create_order` according to the documentation.

Prevents exceptions from being raised when the request results in a status code other than 2xx

## How to test

Set up an API connection according to the README

In an irb console:
`require_relative 'path/to/project/lib/bigcommerce'`

set an instance variable (like `order_hash`) to the hash below. Then issue the commands:
`api.create_order(order_hash) #=> returns order_id`
`api.update_order('order-id-from-above-command', {"products" => [{"product_id" => 101,"quantity" => 1}],"external_source" => "ecomm"} )`

```ruby
{                                                                             
      "customer_id"=> 0,                                                          
      "order_is_digital"=> false,                                                 
      "staff_notes"=> "I'm a staff note!",                                        
      "customer_message"=> "HI! I'm from a customer!",
      "billing_address"=> {                                                       
        "first_name"=> "Trisha",                                                                                            
        "last_name"=> "McLaughlin",                                               
        "company"=> "",                                                            
        "street_1"=> "12345 W Anderson Ln",                                       
        "street_2"=> "",                                                          
        "city"=> "Austin",                                                        
        "state"=> "Texas",                                                        
        "zip"=> "78757",                                                          
        "country"=> "United States",                                              
        "country_iso2"=> "US",                                                    
        "phone"=> "",                                                             
        "email"=> "elsie@example.com"                                             
      },                                                                          
      "shipping_addresses"=> [                                                    
        {                                                                         
          "first_name"=> "Custy",                                              
          "last_name"=> "McWallethaver",                                                
          "company"=> "",                                                         
          "street_1"=> "566 Sussex St",                                           
          "street_2"=> "",                                                        
          "city"=> "New York",                                                    
          "state"=> "NY",                                                         
          "zip"=> "10013",                                                        
          "country"=> "United States",                                            
          "country_iso2"=> "US",                                                  
          "phone"=> "",                                                           
          "email"=> "el@example.com"                                              
        }                                                                         
      ],                                                                          
      "products"=> [                                                              
        {                                                                         
          "product_id"=> 102,                                                     
          "quantity"=> 2                                                          
        }                                                                         
      ],                                                                          
      "external_source"=> "the internet"                                          
    } 
```