Eliminate race condition during processDocuments call with multiple URLs

This is in regards to http://forums.zotero.org/discussion/27304/save-multiple-items-to-zotero-on-ieee-xplore-search-results/

Here's what was happening:
1) Queue up two URLs for processDocuments
2) URL one is being processed via `loadURI`, `loaded` is set to false. onLoad is called, but the `doc.readyState` is still loading. 1 second timeout is set to re-run the onLoad function.
3) In the mean time, the page is done loading and another `pageshow` event is fired and onLoad is called. The page loads, `loaded` is set to true, and is passed to the translator. The translator calls doGet/doPost, which results in processDocuments proceeding to the next URL.
4) URL two is processed via `loadURI`, `loaded` is set to false.
5) The 1 second timer from URL one runs out and calls `onLoad` for URL one. `loaded` is set to true and document from URL one is passed back to the translator.
6) URL 2 page loads, but `loaded` is already set to true, so it is discarded.

I'm not entirely sure why we need the 1 second timeout, since it seems that once the page is loaded, a `pageshow` event is fired anyway. I assume @simonster had a good reason for that. So the solution here is to not have a global `loaded` variable, but to use something attached to each browser.

There's still something that puzzles me. I had some debugging code in place that would tell me what the browser ID is for the `e.currentTarget`. Here's a segment of the log from the `loadURI` call of the first URL:
```
(3)(+0000004): Created hidden browser (0)
(3)(+0000004): Created hidden browser (1)
(3)(+0000000): Zotero.HTTP.processDocuments: Loading http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=6378758&contentType=Conference+Publications&searchField%3DSearch_All%26queryText%3Delectrode
(3)(+0000001): Entered onLoad
(3)(+0000001): Browser ID: 0
(3)(+0000081): Entered onLoad
(3)(+0000000): Browser ID: 1
(3)(+0003418): Entered onLoad
(3)(+0000000): Browser ID: 0
(3)(+0000000): still loading. Will try again in 1 second.
(3)(+0000205): Entered onLoad
(3)(+0000000): Browser ID: 0
(3)(+0000000): Zotero.HTTP.processDocuments: http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=6378758&contentType=Conference+Publications&searchField%3DSearch_All%26queryText%3Delectrode loaded
```
Note the `(3)(+0000000): Browser ID: 1` in there. Why is there a `pageshow` event reaching browser 1 before it has any `loadURI` calls initiated on it?? Do the `pageshow` events just happen randomly??