Identity v3 Authentication With TrustID (#24)

* delete auth_results

* v3 auth with trust

* define auth errors in gophercloud pkg

* AuthOptionsBuilder interface

* combine error files in gophercloud pkg
diff --git a/openstack/identity/v3/extensions/trusts/requests.go b/openstack/identity/v3/extensions/trusts/requests.go
new file mode 100644
index 0000000..999dd73
--- /dev/null
+++ b/openstack/identity/v3/extensions/trusts/requests.go
@@ -0,0 +1,34 @@
+package trusts
+
+import "github.com/gophercloud/gophercloud/openstack/identity/v3/tokens"
+
+type AuthOptsExt struct {
+	tokens.AuthOptionsBuilder
+	TrustID string `json:"id"`
+}
+
+func (opts AuthOptsExt) ToTokenV3CreateMap(scope map[string]interface{}) (map[string]interface{}, error) {
+	return opts.AuthOptionsBuilder.ToTokenV3CreateMap(scope)
+}
+
+func (opts AuthOptsExt) ToTokenV3ScopeMap() (map[string]interface{}, error) {
+	b, err := opts.AuthOptionsBuilder.ToTokenV3ScopeMap()
+	if err != nil {
+		return nil, err
+	}
+
+	if opts.TrustID != "" {
+		if b == nil {
+			b = make(map[string]interface{})
+		}
+		b["OS-TRUST:trust"] = map[string]interface{}{
+			"id": opts.TrustID,
+		}
+	}
+
+	return b, nil
+}
+
+func (opts AuthOptsExt) CanReauth() bool {
+	return opts.AuthOptionsBuilder.CanReauth()
+}