Memory Leaks in V8StringToChar and its invocations

Following on the issue https://github.com/appjs/appjs/issues/382 I have used:
1. std::unique_ptr instead of raw pointers.
2. Invocations of V8StringToChar has been changed to either get() or release(), depending upon what i thought was required at that position.

I first tried with shared_ptr but since we do not need many pointers to point to this memory we should use unique_ptr. Hence moved to it.