Various small enhancements to cl-interval

Hi Ryan,

first let me thank you for cl-interval! While reading through its source, I added a few
modifications and enhancements which I hope will be uncontroversial and perhaps find
it back to your tree. :-)

I'd also appreciate a clarification of the license. The only evidence I could find is in the
ASDF file which says "NewBSD, LLGPL."

May I ask you to decide on one license (for my purposes, preferably New BSD) and add
an explicit LICENSE file to the project, please? I will happily add the file myself and create
a pull request after your feedback.

Thanks again,

Tobias