symbolize keys in data responses by default

fixes #26 

Convert keys in hashes in response data to symbols.

Old behaviour can be enabled by:

```ruby
Flapjack::Diner.return_keys_as_strings = true
```

```ruby
if return_keys_as_strings.is_a?(TrueClass)
```
if you want to explicitly test for a true value. Also, I'm not sure about the naming of the `sausage` variable... and you'd want to comment out or remove those `puts` statements.