null for expired items, null in mget results, error on invalid maxAgeMs

Hello @x-ma,

Please review the following commits I made in branch 'artem-mget-undefined'.

67509410f0cc956801d6f8b4ca5c5011831f3326 (2013-08-12 20:04:39 -0700)
null for expired items, null in mget results, error on invalid maxAgeMs
if an item is expired or doesn't exist, get/mget returns null

if an array of `n` items are passed into `mget`, an array of `n` results
will be returned with `null` as the placeholder for items that are either
not in the cache or have expired.

if the maxAge of an item was not overriden in a InMemoryCache, or it
was not positive, `set` and `mset` will throw an error saying so.


R=@x-ma