rename `wp site-meta` to `wp network-meta`
rename `wp blog` to `wp site`
alias `wp blog` to `wp site`
behat: fix help --gen test
site create: rename --site_id= to --network_id=
site create: alias --site_id= to --network_id=
Merge pull request #523 from wp-cli/blog-site-network

Rename `blog` to `site` and `site-meta` to `network`