Added onUserSignupBlock to A0LockViewController

[liamnichols]

Hey,

Due to limitations with the Sign Up UI in the A0LockViewController, we had been unable to use it because we need to capture additional fields however the client still wanted a way to access the Sign-Up interface from within the A0LockViewController (so that the users don't miss things like social sign in).

I come up with a solution to add a onUserSignupBlock block (similar to onUserDismissBlock) allowing developers to provide an override point upon the user selecting the Sign Up button.

If nil (default), the SDK will work like it did previously and present the interface provided by the app however if the developer sets this block, upon the user selecting the Sign Up button, the block will be executed instead.

This means that we can do something like the following:

A0LockViewController *controller = [[A0LockViewController alloc] initWithLock:lock];
controller.closable = YES;
...
controller.onUserDismissBlock = dismissBlock;
controller.onUserSignupBlock = ^{
    [self presentSignupScreen];
};

Would be great to get it merged back into the main branch so that we can still receive other updates without having to manually merge into our own fork.

Thanks

[hzalaz]

@liamnichols thanks for the PR. We have a couple of request about custom fields for Sign Up but didn't found a good way to include that in Lock UI.

I am not to keen of start adding multiple blocks to customise the navigation of Lock, we usually recommend to build a custom UI in these cases.
This is our guide that shows you how to build a custom UI, and even has a sample written in Swift and is not that hard if you only have DB and Social connections since it's just calling the correct authentication, the most complicated part we have in Lock is how to handle enterprise connections like ActiveDirectory and such.

Let me think about this PR to see if we can improve it a little bit 😄

[hzalaz]

@liamnichols I am closing this in favor of #183. It's already merged and released in CocoaPods. It has almost the same API but instead of handling yourself the presentation, Lock will handle it and provide a way to navigate back to it. (The PR has a little more info till we write the docs)