Make sure temporary course directory uses safe characters.

Code to convert course_ids to filenames is handled outside the opaque keys library.  

Here is one relatively insignificant place, but one that is needed to support export for researchers.  Hopefully this matches the general we want to take in the LMS and analytics.  @nasthagiri @cpennington @clintonb 
My original motivation was to prepare for the time when Studio permits course_ids that contain non-ascii characters, given that opaque keys currently permits such characters.  And once that happens, I don't see a reason why authors would limit themselves to an occasional accent -- I would expect the entire identifier to be non-ascii.  So if we stripped filenames to ascii-only, then all these non-ascii course-ids would end up looking like a string of hyphens.

On the other hand, I don't know what filesystem Nimisha is using, but I'm pretty concerned about consistency in unicode support across platforms.  Ironically, one of the sources of this is a blog posting by @nedbat (http://nedbatchelder.com/blog/201106/filenames_with_accents.html).   So while Studio still prevents authors from creating courses with such characters in their course_id, this is not an issue, and we can revisit the issue at that time when there are likely to be many other places in the code that would need to be fixed.  So I'm going to remove the unicode flag above (and in the pipeline as well).