if hash is not character, cache does not exist
cosmetic
cache$output() does not check if the cache object exists, just load it with the expected mode

because I will check cache$exists() before loading it; this should not be a problem unless the user did something really bad (e.g. hacked the cache database)
closes #396: cache = 0, 1, 2, 3 for different levels of cache

I think this also closes #536
bump version; prepare CRAN release now
update evaluate and knitr versions
narrower code in the example knitr-twocolumn.lyx