oauth2: add support for client credential grant type

Creates a new package called clientcredentials and
adds transport and token information to the internal
package. Also modifies the oauth2 package to make
use of the newly added files in the internal package.

The clientcredentials package allows for token requests
using a "client credentials" grant type.

Fixes https://github.com/golang/oauth2/issues/7

Change-Id: Iec649d1029870c27a2d1023baa9d52db42ff45e8
Reviewed-on: https://go-review.googlesource.com/2983
Reviewed-by: Burcu Dogan <jbd@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

internal/oauth2.go

@@ -67,3 +67,10 @@ func ParseINI(ini io.Reader) (map[string]map[string]string, error) {
 	}
 	return result, nil
 }
+
+func CondVal(v string) []string {
+	if v == "" {
+		return nil
+	}
+	return []string{v}
+}