Bug fixes and improvements

This release brings stability and improvements. It contains some minor BC-breaking changes which should not affect general public. See full change log below for details

Changelog:

* - BC-breaking changes

• * require libv8 >= 6.4.6;
• * remove UnboundScript::kNoScriptId const and return null in getId() on no data
• * change ScriptCompiler::cachedDataVersionTag(): int method name and return type to be ScriptCompiler::getCachedDataVersionTag(): float;
• * remove StartupData::getRawSize() method;
• fix segfault when invalid startup data passed to isolate;
• fix external exception lost when it has refcount 1;
• change script compiler version tag generation to take into account changes to extension internals;
• add CallbackInfoInterface;
• add support for ScriptCompiler::kProduceFullCodeCache;
• add {Isolate,Context}::within() as optimization solution;
• add StartupData::isRejected();
• add docs skeleton.