feat(ngTap): add ngTap directive for fast clicks on touch devices

Add a new module ngMobile, with mobile/touch-specific directives.
Add ngTap, equivalent to ngClick but using touch events, which are much
more responsive on mobile.

Note that this PR is incomplete. It lacks docs, and the tests aren't passing locally and haven't been tried on CI. The (pretty basic) tests for ngTap succeed on jqLite but fail on real jQuery, and I don't know why.