Implement Twitter Provider

[samtstern]

For the next major version we should attempt to add Twitter authentication alongside Google and Facebook.

This was done in Android on a feature branch in this PR:
firebase/FirebaseUI-Android#268

[dakeshi]

Good news.
In twitter implementation, there are two options to use Twitter API keys.
(reference: https://docs.fabric.io/apple/twitter/advanced-setup.html#url-scheme)

• using info.plist(recommended way)
• setting the Consumer Key and Secret in the code

I think that the former looks more simple. The latter needs the code below
(The former don't need this code):

Twitter.sharedInstance().startWithConsumerKey("your_key", consumerSecret: "your_secret")

[yramanchuk]

Added integration with Twitter provider (v0.5.6).
You need to start Fabric and provide twitter key/secret in info.plist