I18n & exampler choice

### New features

- Choose your preferred exampler with `X-Swaggerific-Responses-From: schema` for preferring the schema, or `X-Swaggerific-Responses-From: examples` (default) for using the examples.
  - If your preferred exampler can't produce the requested content type (eg. Schemas can only produce `application/json`) then it'll fall back on the other examplers or send back a `501` if none are available.
- Fixes #21: prevents locale changes from bleeding between requests