Feature/block cache

Magento only has block caching for top menu (catalog navigation) and the footer.

This branch works in order to add block caching to other pages.

Very rudimentary tests suggest that the work so far can save up to 20% dispatch time on product pages.

In most cases this entails adding cache_lifetime, cache_tags and cache_key info. 

For sales_quote related items I've added a cache tag to the model and also a cleanCache call in beforeSave.  This will then delete the cache for certain quote (by its cache tag)