Projects / Lithium

### The Problem
- We were running multiple Lithium apps using the same cache backend (such as Apc or Memcache) and needed a way to keep cache keys separated between apps.
- I see \storage\cache\adapter\Apc has a 'prefix' in the default configuration, but I couldn't find it being used anywhere or figure out how to override it.
- Obviously, we (or anyone) could subclass Cache to add this functionality.  However, I think this is going to be a common problem and it takes some poking around to do it properly (still handling multi-reads and writes was a gotcha I encountered - thank you unit tests).

### My Solution
- The Cache::key() method is called without knowing which configuration it is for.  Therefore, to add prefix support, I had to resort to looking in the 'default' cache configuration for a 'prefix' key.  I had to make the Cache::key() method a bit more verbose in order to handle all use cases.
- I updated to Cache docblock and added to \tests\cases\storage\CacheTests.php
- Code can be merged from the master.cache-prefix branch on my rmarscher form.
- Alternatively you can cherry-pick 03729b29d1c75a3556a39d1f9ea4ce2a26fa1b34