Add GitHub and Bitbucket authentication

I've added GitHub and Bitbucket authentication through OmniAuth. To check it out you will need to set up developer apps on GitHub and Bitbucket (it's free). Then you need to add an .env file in your project root with the following content:

```ruby
GITHUB_KEY=your_github_key
GITHUB_SECRET=your_github_secret
BITBUCKET_KEY=your_bitbucket_key
BITBUCKET_SECRET=your_bitbucket_secret
PORT=3000 # Optional, foreman uses 5000 by default, I prefer 3000
```

These environment variables are used in the OmniAuth initializer. After starting the server with `foreman start`, you can authenticate by going to [/auth/github](http://localhost:3000/auth/github) or [/auth/bitbucket](http://localhost:3000/auth/bitbucket).