Add an AMD version of shepherd.js

I noticed that including shepherd.js using require.js would result in it returning Tether, which is included by default in the build. Tether is packaged with the resulting shepherd.js during build phase, and being AMD compatible returns itself when run as part of shepherd.js that is built with the gulpfile.

These changes add a second build pathway to the gulpfile, which produces a file shepherd-amd.js, which does not include the code for Tether and wraps only the Shepherd JS with AMD compatible code, which has Tether as a dependency.

Thanks for taking a look!