net/http/cookiejar: add way to access all cookies in CookieJar

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

1.6.3

What operating system and processor architecture are you using (go env)?

darwin / amd64

What did you do?

Tried to access all cookies stored in a http.CookieJar.

What did you expect to see?

A Function for accessing the []*http.Cookie data. The cookiejar struct is missing a way to access all the cookie structs saved in it. You have to know all the matching urls (as url.URL structs) in order to pull cookies out of it.

What did you see instead?

Only a method for reading cookies from pre-known url.URL structs.