Adds os-server-groups

This commit adds support for the os-server-groups extension. This allows
users to create scheduling policies for launching instances.
diff --git a/openstack/compute/v2/extensions/servergroups/results.go b/openstack/compute/v2/extensions/servergroups/results.go
new file mode 100644
index 0000000..d74ee5d
--- /dev/null
+++ b/openstack/compute/v2/extensions/servergroups/results.go
@@ -0,0 +1,87 @@
+package servergroups
+
+import (
+	"github.com/mitchellh/mapstructure"
+	"github.com/rackspace/gophercloud"
+	"github.com/rackspace/gophercloud/pagination"
+)
+
+// A ServerGroup creates a policy for instance placement in the cloud
+type ServerGroup struct {
+	// ID is the unique ID of the Server Group.
+	ID string `mapstructure:"id"`
+
+	// Name is the common name of the server group.
+	Name string `mapstructure:"name"`
+
+	// Polices are the group policies.
+	Policies []string `mapstructure:"policies"`
+
+	// Members are the members of the server group.
+	Members []string `mapstructure:"members"`
+
+	// Metadata includes a list of all user-specified key-value pairs attached to the Server Group.
+	Metadata map[string]interface{}
+}
+
+// ServerGroupsPage stores a single, only page of ServerGroups
+// results from a List call.
+type ServerGroupsPage struct {
+	pagination.SinglePageBase
+}
+
+// IsEmpty determines whether or not a ServerGroupsPage is empty.
+func (page ServerGroupsPage) IsEmpty() (bool, error) {
+	va, err := ExtractServerGroups(page)
+	return len(va) == 0, err
+}
+
+// ExtractServerGroups interprets a page of results as a slice of
+// ServerGroups.
+func ExtractServerGroups(page pagination.Page) ([]ServerGroup, error) {
+	casted := page.(ServerGroupsPage).Body
+	var response struct {
+		ServerGroups []ServerGroup `mapstructure:"server_groups"`
+	}
+
+	err := mapstructure.WeakDecode(casted, &response)
+
+	return response.ServerGroups, err
+}
+
+type ServerGroupResult struct {
+	gophercloud.Result
+}
+
+// Extract is a method that attempts to interpret any Server Group resource
+// response as a ServerGroup struct.
+func (r ServerGroupResult) Extract() (*ServerGroup, error) {
+	if r.Err != nil {
+		return nil, r.Err
+	}
+
+	var res struct {
+		ServerGroup *ServerGroup `json:"server_group" mapstructure:"server_group"`
+	}
+
+	err := mapstructure.WeakDecode(r.Body, &res)
+	return res.ServerGroup, err
+}
+
+// CreateResult is the response from a Create operation. Call its Extract method to interpret it
+// as a ServerGroup.
+type CreateResult struct {
+	ServerGroupResult
+}
+
+// GetResult is the response from a Get operation. Call its Extract method to interpret it
+// as a ServerGroup.
+type GetResult struct {
+	ServerGroupResult
+}
+
+// DeleteResult is the response from a Delete operation. Call its Extract method to determine if
+// the call succeeded or failed.
+type DeleteResult struct {
+	gophercloud.ErrResult
+}