This adds back the memory tests which ensure The Ruby Racer's
stability. It also includes the changes needed to make these initial
specs pass.
At this point to disallow calling Dispose() directly on the underlying
Isolate. If you do, then you could have all of the Ruby objects happily
sitting in memory, but trying to use them would result in a segfault for
trying to use a dead isolate. In order to avoid that, we'd have to put
in safeguards everywhere to make sure for any operation on any context
or object, that it's isolate was still alive.
In order to reduce the complexity of memory management, the safest thing
is to let the garbage collection hook dispose of the isolate
itself. That way there is no doubt that the isolate is no longer in use.
