Key WPBE user meta for compatibility with multisite

If multisite, add the site ID to the user meta key. This enables
a user to have different email notification settings for each
site.

Renames `$options_key` to be the more descriptive `$user_meta_key`

Closes #6
Add language to specify this is a site specific setting
Merge pull request #10 from jeremyfelt/per-site

[PR] Key WPBE user meta for compatibility with multisite
Props for #10