support for multiple parameter values

changes:
- disallow null for parameter values (Play JSON deserializer will enforce this behavior, unlike json4s).
- collapse sending multiple cookies in a single header (RFC 6265)
- send multiple parameter values to the backend
  - query : `a=1&a=2`
  - cookie : `a=1; a=2`
  - header : duplicates, `a: 1` and `a: 2`
  - path, body : not supported
  - warts :
    - Order is kept when sending to the backend, but does not influence the cache key
    - Parameter versioning only consider the first value
- Normal parameters do not override meta parameters anymore (they cannot, nor should they ever)
- Java client : use a Seq.builder instead of a var.

fixes #20 
Right. When I said "now a thing of the past as far as I can tell", what I meant was that we no longer really care if Swagger handles it (well) :laughing:  

I mean, it's not like we 1. use the codegen 2. actually have any hope of making the types display well even without the use of map (look at all the options, durations and enums fail to resolve properly)