re-added #pragma once guard to CinderAssert.h

A while back we removed the include guard from CinderAssert.h so that individual compilation units could define how assertions were handled.  However, in practice this turned out to be a problem, specifically when we wanted to direct the failed assertions in lib code to a specific handler in our app.  This just wasn't possible unless we created a completely new set of assertion handlers that wrapped `CI_ASSERT` and friends.

So this re-adds the guard.  Now if you want custom functionality, you can define it in your build settings and all failed assertions are handled in a uniform manner.  So if you define `CI_ENABLE_ASSERT_HANDLER` and a custom `ci::assertionFailed()` method in your app, even code within cinder will route the failure message to this function.