Issue 1280 - Update v8

IMPORTANT: not tested for Android.

* it does not work on MSVS2010, such fixes will be in another commit
* update to the most closest revision of maxthon. Despite they said it's
3.20.16, the closest version is 062a08f5e129ea0fdc0376a6cd51de7570294682
* adapt code to work with the current v8 version.

V8ValueHolder
- adapt for the new v8::Persistent syntax
- add typedef typename v8::Persistent<T> V8Persistent;
- remove copy-ctr, because it's not used, but requires support
- remove V8ValueHolder(v8::Isolate* isolate, v8::Persistent<T> value) because
it's not used and currently it's not possible to copy v8::Persistent.
- adapt V8ValueHolder::reset methods
- remove type conversion operators because they are not supported by current
v8::Persistent

JsValue:
- pass JsValuePtr as const JsValuePtr&. It's a performance optimization, avoids
unnecessary interlocked calls.
- add v8::Local<v8::Value> JsValue::unwrapValue() const; It helps to retrieve
v8::Local from v8::Persistent because the syntax has been changed.
- adapt to the new syntax.

JsContext, JsEngine, FileSystemJsObject
- adapt code, nothing important