Updated site url to read from `ghost:site` meta tag on theme
Updated site url to read from `ghost:site` meta tag on theme

closes https://github.com/TryGhost/members.js/issues/39

- Script needs correct ghost site url to initialize correct API endpoints for fetching site/members data
- `window.location.origin` won't work as correct ghost site url in all cases, specially for sites using subdirectory setup
- New meta tag `ghost:site` is inserted on themes with correct `siteUrl`, which is used to initialize api

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost