Factor out ensure_all_started into webmachine_util

We have a special version of ensure_all_started that uses the
default Erlang implementation if it's available, and otherwise falls
back to our own reimplemented version of the function.

This is useful to have in places other than just the one test function,
so I'm factoring this out into the webmachine_util module.
Fix webmachine_demo start: use ensure_all_started

Previously we were getting a crash on start due to ssl not being
started. Using ensure_all_started fixes this, and also allows us to
remove some of the boilerplate code for manually starting other
application dependencies.