Fix memory leak while destroying the viewer.

Hi,

I've fixed a memory leak in the destroy() method of the viewer.

Screenshot before (leaking 63MB!)
![mem_check_bug](https://cloud.githubusercontent.com/assets/1844657/3308452/49351782-f67f-11e3-9dd1-a156eddacf78.png)

Screenshot after (actually it's not leaking anything now: the 55KB reported are internal objects created at run-time the first time the code is executed, the following calls to create/destroy are not leaking anything).
![mem_check_fixed](https://cloud.githubusercontent.com/assets/1844657/3308453/4c4c2fa0-f67f-11e3-899e-ddb6016a1350.png)

Let me know if you have any trouble with this fix.
(Fill free to remove the memory_check.html or to point to a local dataset instead).

Thanks,
--Henri
"Necessary" should be "necessarily".