Fix security flaw and add Soft(optional) authentication feature

[Arinzeokeke]

Hi, here are my modifications in detail

1. Security flaw in authenticate_for.

• Using authenticate_for directly for namespaced models(any model actually) doesn't send back an Unauthorized header when an invalid/no token is sent as authenticate_for doesn't check for that.

• I added wrapper functions set_authenticate_for and set_soft_authenticated_for to fix that issue for strict and optional authentications respectively.

2. Soft (Optional) Authentication

• Some controllers may not require authentication but acts slightly different if authenticated.
  Example: It may add extra(private or user-specific) values to JSON response if authenticated.

• Modified method_missing in authenticable.rb to accept the soft_authenticate_<entity> method to implement said behaviour.

• Also added set_soft_authenticate_for for same functionality as explained in 1.

3. Added tests to ensure changes are working

4. Updated README

• Updated readme to reflect changes