bake: Changes in how assets directories are configured. Change `skip_patterns` and `force_patterns` to `ignore` and `force`. Put less responsibility on the `bake` command to specify all those settings, and more on the `Baker` and `ProcessorPipeline` themselves. Add some tests.

#!/bin/sh

CUR_DIR="$( cd "$( dirname "$0" )" && pwd )"
CHEF=${CUR_DIR}/../bin/chef
OUT_DIR=${CUR_DIR}/../piecrust/resources/messages
ROOT_DIR=${CUR_DIR}/messages

$CHEF --root=$ROOT_DIR bake -o $OUT_DIR
rm ${OUT_DIR}/index.html