Cache api for non-holdbacks endpoints

Re-introduce caching on the gem level. You can specify any class that inherits from Viki::Core::Base to be cacheable with the following

```ruby
class Viki::User < Viki::Core::Base  
  cacheable
  ..
  ..
end
````

This will only effect the fetch request.

Default cache expiry time is set to 5 seconds. It can be configurable.