Try loading NSS by simple name first (fixes issue #4)

On Fedora 18 (and possibly other GNU/Linux distributions), the NSS library is not located in `/usr/lib/firefox/xulrunner/` which is the path returned by `Services.dirsvc.get("GreD", Ci.nsILocalFile)`. The library can be loaded when only its name is provided.

This commit patches the initialisation code such that the simple name is tried first. If the library cannot be found, the full path is tried instead.