src: kill isolate on exit

Otherwise it's not possible to check from inside a destructor if V8 is
still alive with v8::V8::IsDead().  In V8 3.25, that function returns
true until the last isolate is destroyed.

This used to work in v0.10 and is a standard trick to dispose persistent
handles conditionally.

Signed-off-by: Fedor Indutny <fedor@indutny.com>