Add optional forgot password link

With this PR, you can add an optional Forgot Password button to the Login ViewController (Ref: #155). This button is only shown when is explicitly activated and an valid URL is supplied using a localized string. By using the localized string, you can set specific URL's per language. This forgot password button switches place with the TOS button, so it doesn't take extra space in the Login ViewController.

To activate the button use: `[SPAuthenticationConfiguration sharedInstance].showForgotPasswordButton = YES;`
The localized string that should be used for the Forgot Password URL is: `"Forgot Password URL"`

This PR also changes the TOS URL to a localized custom TOS URL using: `"TOS URL"`. if it's not supplied and/or valid, it uses the default Simperium TOS.

Hope it helps!

Regards Patrick