rely on --path, instead of using chdir()
add core download check in 'Empty dir' scenario
use ABSPATH in `core download`, instead of '.'
create ABSPATH if it doesn't exist. props @erwanlr
Ignore `wp ` prefix straigth from step definition.

This makes it clearer that running arbitrary commands is not supported.
Prepend `--path`, instead of appending it, so that piping works.
Merge pull request #327 from wp-cli/fix-core-download

Fix path handling in `core download`