support AMD

Hi Rico

My colleague used ractive-touch on a project the other day ([this article](http://www.theguardian.com/world/ng-interactive/2014/aug/20/-sp-cemetery-for-our-people-guatemalan-consul-texas-migrant-crisis), scroll down to 'The things they carried') - it worked beautifully and saved us a ton of time, so thank you.

We had to modify it to work with our AMD-based stack, so I thought I'd submit this PR - getting modules with external dependencies to work across browserify/AMD/normal browser is a massive pain but this pattern seems fairly reliable.