Remove redundant check for duplicate embed

This check is in the inline script that gets added, so there's no need
to have it here.

And set `window.annotator` explicitly, just for good measure.
whitespace
Improve duplicate embed prevention

Set `window.hypothesisHasRun` rather than trying to detect duplicate
embeddings in the background script. For the extension this should be
very reliable because the symbol will be isolated to the content script
context and can never conflict with variables set by page authors.

Don't evaluate the yepnope script except when it is actually needed.
Wait for PDF.js to render before embedding

Seems reliable enough through manual testing but it's hard to be sure
that there's no edge case where it fails.

I'm currently unable to reproduce on large or small PDFs on both
residential broadband and slow connections on a train and 3G tether.

Close #893.