Supporting OPTIONS requests

[carllerche]

Making OPTIONS requests has turned out to be problematic.

OPTIONS requests can include a "path" component of *. These requests must include a :scheme as well as optionally include a :pseudo header (spec).

Now, the problem is that there isn't a great way to represent these possibilities using the Uri type provided by http.

This also relates to hyperium/http#176.

This is a continuation of this discussion.