refactor(NLVObject): use smart pointers for tracking object lifetime

[Rush]

@mbroadst I had a lazy evening and a crazy idea to use weak pointers instead of NLVObjectPtr. Note: enable_shared_from_this is probably not necessary but I wanted you to see this branch earlier rather than later to get some comments.

[mbroadst]

yeah definitely now that we've committed to c++11, smart pointers are of course the way to go :)

[mbroadst]

@Rush I left a few comments. I really like the direction this is taking, but I wonder if we can't split the single commit into at least two:

• one for the move to std::unique_ptr + all of the handle_ => handle() changes
• one for your idea with std::shared_ptr

It's a bit hard to review this mono-commit version because of all the handle_ => handle() noise, and frankly I would merge that one right in because it's pretty straightforward and obviously the better solution.

Thoughts?

[Rush]

Sure, I'll split it later this evening probably. Thanks for leaving the comments.