Go 1.7

[ejholmes]

Go 1.7 brings native support for context.Context, most importantly within net/http. We should make appropriate changes to pkg/httpx to use the standard http.Handler interface.

The major advantage will be that we can use any http.Handler middleware/router (e.g. negroni, gorilla mux), without having to re-implement it to support the httpx.Handler interface.

Also, for outgoing requests, http.Request now natively supports cancellations from the context (e.g. _ cancel := context.WithCancel(req.Context()); cancel()) so we no longer have to re-implement http.Client.