Add example that makes sure it works everywhere

examples/os.rs

@@ -0,0 +1,27 @@
+extern crate users;
+use users::{Users, Groups, UsersCache};
+use users::os::unix::UserExt;
+
+fn main() {
+	let cache = UsersCache::new();
+
+	let current_uid = cache.get_current_uid();
+	println!("Your UID is {}", current_uid);
+
+	let you = cache.get_user_by_uid(current_uid).expect("No entry for current user!");
+	println!("Your username is {}", you.name);
+	println!("Your shell is {}", you.shell().display());
+	println!("Your home directory is {}", you.home_dir().display());
+
+	let primary_group = cache.get_group_by_gid(you.primary_group).expect("No entry for your primary group!");
+	println!("Your primary group has ID {} and name {}", primary_group.gid, primary_group.name);
+
+	if primary_group.members.is_empty() {
+		println!("There are no other members of that group.");
+	}
+	else {
+		for username in primary_group.members.iter() {
+			println!("User {} is also a member of that group.", username);
+		}
+	}
+}