Minor Cleanup

Hi, I have just been checking out your Firefox extension -- looks interesting; thanks :-).  It seems that some hidden files from Finder on a Mac have been committed to the repository.  Here is a commit that removes them and will stop them from re-appearing in future commits from Mac users; I hope you'll find it of use.

* Removed Finder (Mac OS X) .DS_Store files.
* Added a .gitignore to ignore the Finder files.
* Set executable permissions on build scripts.