[jshint] Bugfix: Load sources to check in sync.

This is a quick-fix to avoid the following situation:

1. $("script").map makes an ajax request to load each source file and stores it in Screw.jshint_scripts.
2. A screw unit test is defined, where each Screw.jshint_scripts is checked against JSHINT.

Since the ajax calls where previously async, JSHINT sometimes already run, when ajax call was not yet finished:

![schnappschuss 2014-03-10 10 55 04](https://f.cloud.github.com/assets/86275/2372402/1e7897c8-a83a-11e3-98b9-cc0d70a897a1.png)

This pull-request avoids the problem. In my local tests I could not see a slowdown larger than 1 second.