fix(datepicker): compatibility with angular 1.1.5 and no jquery

Fix for #760

As I'm pretty noob on good GitHub contrib, don't hesitate to make comments.

There was no easy way to make unit tests in the bug conditions (no jquery and angular 1.1) so I simply verified that it doesn't breaks existings ones.

For the initial `$document.unbind` which failed, I didn't found better than using a boolean preventing unbind before first bind.

For the `open` attribute problem, as I understand this new directive : [ngOpen](http://code.angularjs.org/1.1.5/docs/api/ng.directive:ngOpen), the open attribute is reserved in HTML5 and can only be a boolean. I therefore proposed to switch to `is-open` with a backward compatibilty trick allowing to use `open` attribute if the angular version allow it (and is-open is not used).

Hope this helps.
Matthieu