Bug fixes

- Switch to https://github.com/atlassian/xmpp4r for better parsing
- Add nokogiri dependency
- Rid of legacy `source :rubygems`
- Add Gemfile.lock to .gitignore
- Use Thread.join instead of looped sleep
- Refactor/fix GithubUser
  - Switch to open-uri from Net::HTTP
  - Change to class methods to avoid overwriting DM-supplied methods