Method to check if a strategy has a provider registered.
Controller that authenticates with a UIWebView.

Added flag to tell `A0AuthenticationViewController` to authenticate with
UIWebView when a social connection is selected.
By default the controller authenticates with a third party app (Safari or
Social iOS app if available).
Color for webview controller bar.
Refactor web authentication.

New object to handle URL callback of web auth.
Renamed Facebook & Twitter helper classes.
Update README.
Update README

[skip ci]
Merge pull request #24 from auth0/feature-webview-authentication

WebView Authentication