Modified: Support multiple user.

Support multiple user.

**Setting attribute.**

```ruby
default["github-pubkey"]["github-members"] = %w[udzura akubi]
default["github-pubkey"]["usernames"] = %w[neet eimingu]
```

**Produce result**

1. Download ssh public key of ```http://github.com/udzura.keys``` and ```http://github.com/akubi.keys```
2. Concat of ssh public key files.
3. To copy concat ssh public key to user directory.