Allow generating example request bodies for multipart/form-data

[kylef]

Request example bodies are now generated for formData parameters while using the multipart/form-data consumes content type.

Dependencies

• [x] https://github.com/apiaryio/swagger-zoo/pull/47

[kylef]

I've updated this to do the following:

• Changed default boundary from md5(boundary) to just BOUNDARY as I think this behaviour is clearer than an obscure hash, however it does increase the risk that the boundary clashes with some example values or keys.
• Moved all media-type specific logic to separate file media-type.js