fix(scroll): ignore events during slide-box slide

I don't expect this PR to be merged. @mhartington [mentioned](http://forum.ionicframework.com/t/beta-14-slide-box-limit-scroll-whilst-sliding/14393/3) there was a refactoring of the slidebox going on, so this probably isn't aligned with those efforts. Nonetheless, I thought it might be helpful for others affected by the "scroll leak" introduced in beta 14.

The details of the bug can be found in various issues on the forum:

- http://forum.ionicframework.com/t/beta14-slide-box-vertical-scroll-behaviour
- http://forum.ionicframework.com/t/beta-14-slide-box-limit-scroll-whilst-sliding
- http://forum.ionicframework.com/t/ion-content-scroll-locking-doesnt-work

The implementation followed the lead of `ionic.scroll.isScrolling` by introducing `ionic.slide.isSliding` so that the two may communicate as necessary. isSliding turns on during a slide view's `touchmove` and off in `touchend`. When isSliding is true, it interrupts the scroll event from a present scroll.

Tests didn't break, but I also didn't add any tests (back to not thinking this would be merged). If I was mistaken, or this is terrible for other reasons I cannot see, please let me know!

Thanks, and Happy Holidays ionic team!