Adding BellyPlatform.env attribute

Added `env` getting / setter to the BellyPlatform module. This uses `ActiveSupport::StringInquirer` which supplies some helper methods to give us a cleaner interface to RACK_ENV. Rails does it this way, so I think it makes sense to steal it.

Now we can do things like:

```
BellyPlatform.env
=> "development"
```

```
BellyPlatform.env.development?
=> true
```

```
BellyPlatform.env = 'foo'
=> "foo"
BellyPlatform.env.foo?
=> true
```

I also updated the scaffold generator to use these new methods.


@jdoconnor @craigulliott