Add expiration times for caching

[rafamanzo]

Currently we are relying on the engine (memcached at mezuro.org) to expire our cache for example at the home#index. This can lead to undefined behaviour and code harder to developers understand.

We should set the expiration options accordingly: http://guides.rubyonrails.org/caching_with_rails.html

[danielkza]

I think it's fine to rely on Memcached expiring keys, as it's part of what it's expected to do, but we just need to make sure we have the same behavior on other environments to avoid surprising differences. That might just mean configuring the cache in the development/test environment, or using explicit timeouts on some things.