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.

Binding.gyp has been changed to work at my Local repo. That does not come as part of these changes.