Update README

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

WebView Authentication
Make only public main VC & Theme classes.
Add class to handle authentication params.
Use A0AuthParameters to send optional parameters to API.
Refactor on how auth parameters are stored.
Send authentication parameters for login/signup/reset password.
A0AuthParameters for social authentication.

Fixed issue with embedded UIWebView auth.
Xcode 6 & Cocoapods 0.34
A0AuthenticationViewController has a default value for
authenticationParameters.

Update README and A0APIClient comments.