fix(Scope): aggressively clean up scope on $destroy to minimize leaks

Due to a known V8 memory leak[1] we need to perform extra cleanup to make it easier
for GC to collect this scope object.

The theory is that the V8 leaks are due to inline caches which are caches
built on the fly to speed up property access for javascript objects.

By cleaning the scope object and removing all properties, we clean up ICs
as well and so no leaks occur.

I was able to manually verify that this fixes the problem for the following
example app: http://plnkr.co/edit/FrSw6SCEVODk02Ljo8se?p=preview

Given the nature of the problem I'm not 100% sure that this will work around
the V8 problem in scenarios common for Angular apps, but I guess it's better
than nothing.

[1] V8 bug: https://code.google.com/p/v8/issues/detail?id=2073

Closes #6794
Closes #6856