adopts ISC license

Hi Eric!

I've taken the liberty of switching this to using the [ISC license](http://opensource.org/licenses/ISC).

Coincidentally, that happens to be the license that the OpenBSD project prefers to use. As [they say](http://openbsd.org/policy.html), "The ISC copyright is functionally equivalent to a two-term BSD copyright with language removed that is made unnecessary by the Berne convention. This is the preferred license for new code incorporated into OpenBSD."

Since you currently hold full copyright, you can readily relicense AyaDN by merging this pull request.

It's far simpler to contribute when a project uses a well-known style of license, and this is even required by the generally accepted meaning of "open source", for which see [Open Source Licenses](http://opensource.org/licenses/).

Thanks!