#4 add server cache

[Mark Porter]

Add a global cache with the following requirements:
- can be set/retrieved via $server.cache.set() and $server.cache.get()
- ability to set a cache lifespan in seconds
- ability to set "refresh" function to be executed by $server.cache.get() when cache is expired
- ability to clear the cache by name ($server.cache.clear("name"))
- ability to manage live server caches from Myna Administrator (view and
clear)
- ability to set maximum number of cached items in Myna Administrator