Add username parameter to user search (#70)

Check out http://docs.gitlab.com/ce/api/users.html

users.go

@@ -73,6 +73,7 @@ type ListUsersOptions struct {
 	ListOptions
 	Active   *bool   `url:"active,omitempty" json:"active,omitempty"`
 	Search   *string `url:"search,omitempty" json:"search,omitempty"`
+	Username *string `url:"username,omitempty" json:"username,omitempty"`
 }
 
 // ListUsers gets a list of users.