add host override for surtax

Does this work with 
```ruby
config.base_url = options[:host_override] unless options[:host_override].blank?
```

#blank? returns true for nil and empty strings (see here: http://api.rubyonrails.org/classes/Object.html#method-i-blank-3F).  It is, however, a rails thing so I'm not sure if it'd work in the gem.