Support user defined LoginViewController in swift3

[hmuronaka]

Hi, I wanted to use CustomizedLoginVIewController instead of original LoginViewController.
SwiftlySalesforce3.0 also supported "user defined loginviewController" previously supported by 2.2.0

[mike4aday]

Thanks @hmuronaka -- I made 2 minor modifications (below) to your code and included it in just-released version 3.1.0.

Modifications:

1. Instead of a new protocol LoginViewControllable I adapted the existing 'LoginViewController' so it's now a protocol, and a new fileprivate class SafariLoginViewController implements it.
2. Instead of a new method createLoginViewController, I let custom LoginDelegates implement their own version of login(url: URL) that doesn't assume a view controller is involved (e.g. in case of a different OAuth flow that might use stored credentials and no login form).