chore(gulp): refactor tasks to ensure all files are copied

Previously, only 16 of the files from the 'templating' dependency were being copied.
I believe this is due to Gulp terminating before all the files had been written.

This refactoring ensures that all streams are complete before gulp exits - you do this by
returning a stream from the task.  This is tricky to do if you have multiple streams in a
single task. In this case we use the event-stream `merge()` function to turn multiple
streams into a single one for returning.

In addition, the clean task is now run using `runSequence`, which decouples it from the
other tasks so that we don't have to add it as a dependency of every task.

Closes #4