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
Am I right, we are providing `Ttl` in our public API of http4s DSL? Is this expected?