Refactor examples to MVVM

[glm4]

This is the first step of Milestone 1. Fixes #89. Includes:

1. Simple implementation of MVVM in the current examples.
   • Added SignInViewModel, SignUpViewModel and HomeViewModel holding business logic and separating it from Views/ViewControllers.

Next step proposal:

• Decouple content providers(network, local, etc) from ViewModels since currently they are very attached to APIClient, UserAPI, etc.

Notes:

• For the examples purpose, ViewModels are talking to the Views via closures. This leaves the implementation open to any kind of binding mechanism or FRP library if needed.

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Risk:

• High

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[germanStabile]

It would be nice to have an example with a view controller with more complex data (i.e: a collection or several) to show the real gain of MVVM :)

[glm4]

@germanStabile about the example with more complex data, I wouldn't add any more examples to the base but I see your point. If someone would like to see other examples I would recommend the feature/RX_Swift branch.

Right now we don't really see the difference because this is part of a bigger set of changes.