cuahsi/wof addition of optional start & end datetime argument

Andy: Here's my first try at doing a pull request. The only file changed is cuahsi/wof/core.py, as we discussed before.

I followed your suggestions: forked ulmo into my own personal repo, pulled that personal remote repo into my local ulmo repo where I had previously made the change to cuahsi/wof/core.py, then pushed to my personal remote repo. I think it did what it was supposed to! Let me know if everything looks ok.