Allow network-wide queries

Closes #13 .
@lukecarbis Sorry I missed that you were taking care of this in this PR when I did https://github.com/wp-stream/wp-stream/commit/5b790f155cc17b8635a02f3c56a482b42a2e4f8e.

I will be sure to use your commit here instead of mine when the conflicts arise.

However, I added `get_current_blog_id()` for good measure, just in case the hooked function was operating at the network level and needs to know that blog ID was disconnected/connected. What do you think?