Future-based Fetch abstraction

[hgzimmerman]

Add a trait where you can specify method, headers, etc... for a given type (the request)
Have an associated type to indicate the response.

A send_fetch_request function that takes an implementer of that trait as an argument and returns a future that returns the associated response type.

[hgzimmerman]

A working start to this exists in a personal project of mine, its just a matter of porting it over and extending it to enable parameterizing some options like CORS.