v8 3.25 branch, added FatalError event

Upgraded to latest stable v8, including much broader need to supply
isolates to v8 functions.  Cache references to external objects.  Added
FatalError event to notify when (for example) v8 runs out of memory.
Added stack overflow regression test.
Merge pull request #15 from oliverbock/master

v8 3.25 branch, added FatalError event