Support custom config args for caches

[sgibbons]

Add support for dict -> kwargs style config parameters to be passed through to custom cache instantiation via the Expander constructor.

Should work more or less like this for a custom sql-backed cache:

exp = Expander( 
    cache = ( 
        SqlCache, { 
            'server': 'localhost',
            'user': 'some_username',
            'password': 'some_password'
        })
)

[sgibbons]

changed from [enhancement] -> [severe] because this really is necessary for reasonable implementations of persistent caches, i.e, some connection information will likely need to be provided if they're backed by something like CouchDB.