Add type classes to encode a query parameter

This PR is not ready yet, I just wanted to use it a proof of concept for #86 and get your feed backs.

Ideally I would use 3 type classes:
 * one to encode a type T to a `QueryParameterValue`
 * one to decode a `QueryParameterValue` to T (with potential of failure)
 * one to describe the parameter key associate to a T

The decode and encode type classes should satisfy basic encoding laws:
 * for all t, decode . encode == success
you right here there is no specific advantage but I am using `Maybe` in almost all projects I am working on as it is simply an improved version of `Option`. http4s is heavily based on scalaz stream, so it will make sense to support scalaz data structures. However, we can overload the function to accept `Option` as well.