add symlink action to mongod_instance provider
require chef/provider instead of individual resources and providers
update tests for new symlink action
allow directory paths for config, logs and data to be symlinks

use case is if I want to manage the data directory with some other automation,
i.e. business requires that I store the mongodb data set on an encrypted file system,
so /data/db is a symlink to some other place.
bump to 0.1.2
add notice to readme

hat tip to @miah_