If using wrong menu name, it should get default menu instead

What I've tried to do is:
- merge your code back to my repository (after your refactoring to upgrade to version 1.1)
- use indent the same as you refactored. I'm not sure that I've ruined the indent again or not T^T. But I compared files before pushing and they have the same indent. If you found it breaks the indent again, please do not merge this commit.
- the real fix is just 2 lines, about "If using wrong menu name, it should get default menu instead" and "cache wp_get_nav_menus". Please see this commit https://github.com/nattyait/voce-cached-nav/commit/972fda4c4d4420d57401ad6f66ee33c4127a4324
- Have been tested on Wordpress 3.6

I found another bug about when delete the post/trash the post, the cache menu is not cleared properly. I'm going to fix it.