Add support for importing users

Here we add basic support for importing users to state.   Ideally we
would be able to import by username, as there is an api to find users by
username[1], but this is not currently exposed by go-gitlab[2]

Towards #36

[1] https://docs.gitlab.com/ee/api/users.html#list-users documents
	/users?username=:username which worked well in manual testing.

[2] https://github.com/xanzy/go-gitlab/blob/master/users.go#L97-L99

Author: richardc

gitlab/resource_gitlab_user.go

@@ -15,6 +15,9 @@ func resourceGitlabUser() *schema.Resource {
 		Read:   resourceGitlabUserRead,
 		Update: resourceGitlabUserUpdate,
 		Delete: resourceGitlabUserDelete,
+		Importer: &schema.ResourceImporter{
+			State: schema.ImportStatePassthrough,
+		},
 
 		Schema: map[string]*schema.Schema{
 			"username": {

website/docs/r/user.html.markdown

@@ -51,3 +51,11 @@ for the user.
 The resource exports the following attributes:
 
 * `id` - The unique id assigned to the user by the GitLab server.
+
+## Importing users
+
+You can import a user to terraform state using `terraform import <resource> <id>`.
+The `id` must be an integer for the id of the user you want to import,
+for example:
+
+    terraform import gitlab_user.example 42